Training Manual Boeing 737-300/400/500-CFM56-3

ATA 28 – Fuel ATA 104 - Level 3 B1/B2

ATA 28
FUEL
Revision 1/ October 2011

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Training Manual Boeing 737-300/400/500-CFM56-3
ATA 28 – Fuel ATA 104 - Level 3 B1/B2

For training purposes only.

Copyright by Jat Tehnika/Solinair Technical Training.

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 Training Manual Boeing 737-300/400/500-CFM56-3
ATA 28 – Fuel ATA 104 - Level 3 B1/B2

TABLE OF CONTENTS 28-40 FUEL FEED SYSTEM …………………………………… 40

28−00 FUEL …………………………………….. 4 ENGINE FUEL FEED SYSTEM
FUEL SYSTEM INTRODUCTION FUEL BOOST PUMPS
FUEL SYSTEM CONTROLS MAIN AND CENTER TANK BOOST PUMP POWER
FUEL QUANTITY INDICATORS CONTROL
PRESSURE REFUELING PANEL BOOST PUMP CHECK VALVES
ENGINE FUEL SHUTOFF VALVE /ENGINE CROSSFEED
28−10 FUEL STORAGE …………………………. 11 VALVE
FUEL TANKS DESCRIPTION ENGINE FUEL SHUTOFF VALVE ELECTRICAL CIRCUIT
FUEL TANK ACCESS PANEL CROSSFEED VALVE ELECTRICAL CIRCUIT
BAFFLE CHECK VALVES ENGINE FUEL SHUTOFF VALVE SWITCH
TANK DRAIN VALVES DEFUELING
CENTER TANK DRAIN VALVES MANUAL DEFUELING VALVE
APU SHROUD DRAIN 28-50 CENTER TANK FUEL SCAVENGE SYSTEM ………. 58
28-20 FUEL VENT SYSTEM …………………… 24 CENTER TANK FUEL SCAVENGE FLOW INDICATOR
(OPTIONAL)
TANK VENT SYSTEM
FUEL SCAVENGE SYSTEM CONTROL LOGIC
VENT SCOOP AND FLAME ARRESTER
FUEL SCAVENGE JET PUMP
FLAME ARRESTER PRESSURE RELIEF VALVE
28-60 FUEL QUANTITY INDICATING SYSTEM …………… 63
28-30 PRESSURE FUELING SYSTEM ………. 29 FUEL QUANTITY INDICATION CIRCUIT
PRESSURE FUELING STATION FUEL QUANTITY SYSTEM BUSSING PLUG AND
FUELING OPERATION TRANSIENT SUPPRESSION DEVICE
FUELING MANIFOLD FUEL QUANTITY INDICATOR
FLOAT SWITCH FUEL QUANTITY INDICATOR TEST
FUELING CIRCUIT MEASURING STICK
OVERWING FUELING PORT FUEL FEED LOW PRESSURE INDICATING SYSTEM
FUEL TEMPERATURE INDICATING SYSTEM

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 Training Manual Boeing 737-300/400/500-CFM56-3
ATA 28 – Fuel ATA 104 - Level 3 B1/B2

28−00 FUEL
FUEL SYSTEM INTRODUCTION General Fuel System Description
The purpose of the fuel system is to store and distributes Electric motor driven pumps and fuel lines deliver
fuel to the engines and APU. fuel from any tank to one or both engines and the
The fuel system consists of: APU. Fuel from boost pumps can also be delivered
• Tank No. 1 and No. 2 (integral wing tanks) through a defueling valve (manual operated on
• Center tank (three cavities) ground only) into fueling manifold for defueling and
• Fuel Vent System transfer.
• Pressure Fueling System A fuel dump system is not installed. A fuel scavenge
• Fuel Feed System and system removes fuel from the center tank and
• Indicating and Warning System transfers it to main tank No. 1 by a jet pump when
The volume of each tank is larger than the full tank fuel the last center tank boost pump switch has been
capacity to allow for expansion and vent space. The surge switched to OFF. Fuel quantity indicators in the flight
tanks are normally empty and have a capacity of 30 U.S. compartment, and fueling quantity indicators at the
Gallons. Fuel is loaded on airplane from a ground source pressure fueling station, indicate the quantity of fuel
through a fueling receptacle in the pressure fueling station. on the airplane. Measuring sticks are installed on the
The tanks can be refueled simultaneously or one after underside of the tank No. 1 and No. 2. only. They
another. Fuel can also be loaded into tanks No. 1 and No. 2 can be used as an alternate method to determine
through over wing fueling ports. The wing tanks No. 1 and the fuel quantity.
No. 2 have to be refueled at first, until the normal maximum Fuel low pressure lights indicate low fuel boost
capacity. The remainder fuel must be filled into the center pump outlet pressure and a fuel temperature
tank. indicator shows the fuel temperature in tank No. 1
on the forward overhead panel.
During starting the APU an additional DC operated
boost pump supplies fuel from tank No. 1.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 1 – Fuel Tanks

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

28−00 FUEL SYSTEM CONTROLS

FUEL QUANTITY INDICATORS

Are installed in the pilots center panel consist of a:

The Trim Adjustment Screws (3)
• Microcomputer and a Liquid Chrystal Display
• Are installed at the forward pedestal area below
indicate the usable fuel of the respective tank
captains hand mike
send the fuel value to the Fuel Summation Unit
• Allow to adjust the capacity value, of the fuel
memories and display faults in the fuel quantity system by
quantity indication components, up to each fuel
error codes 0−9
quantity indicator
NOTE: The fuel quantity indicators for tank No. 1 and No. 2 in
Note:
the cockpit are identical and fully interchangeable with the
The Trim Adjustment Screws allow to change the
indicators No. 1 and No. 2 at the pressure fueling station.
fuel quantity indicators without indicator adjustment.
The center tank indicators can also be interchanged between
cockpit and fueling station.

The Fuel Summation Unit

is installed at the right hand side over the first officers seat
added the fuel values of the fuel quantity indicators and send
the signal to the Flight Management Computer System
(FMCS)

The Control Display Units (CDU) (2)

• Are installed in the forward pedestal
• Receives and displays data from FMCS
• Displays the actual added fueling signal

The Fuel Quantity Test Switch

is installed at the right hand side of the center tank indicator
checks the fuel quantity indicators simultaneously when
pushing .

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 2 –Fuel Quantity

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

The Engine Fuel Shutoff Valve Position Indicating Light (2)

The Master Caution Fuel Light
• indicates the position of the Engine Fuel Shutoff Valve
• at the Annunciator panel will illuminates when
OFF − Valve open
both Fuel Low Pressure Indicating Lights of a
ON (bright) − Valve in transit or disagree between valve and
tank are ON.
switch position
ON (dim) − Valve in close position
Exception: One center tank boost pump
The Fuel Temperature Indicator
switch is in OFF position and the second
• indicates fuel temperature of tank No. 1
center tank Fuel Low Pressure Indicating Light
The Crossfeed Valve Position Indicating Light
comes on.
• indicates the position of the Crossfeed Valve
OFF − Valve closed
ON (bright) − Valve in transit or disagree between valve and
switch position
ON (dim) − Valve open
The Crossfeed Valve Switch
controls the Crossfeed Manifold Valve connects the engine No.
1 and No. 2 fuel feed lines in order to supply engine No. 2 out of
the tank No. 1 for example, or for fuel transfer on ground
The Fuel Boost Pump Switch (6)
• turns ON the fuel boost pumps in position ON
The Fuel Low Pressure Indicating Light (6)
• monitors the fuel output pressure of the respective fuel boost
pump
ON − Low pressure ( defective boost pump or low fuel level in
the tank ) or fuel boost pump switch in OFF position
OFF − Respective boost pump switch in ON and pump delivers
sufficiently fuel pressure
NOTE:
The low pressure lights of the center tank do not illuminate, when
the respective fuel boost pump switch has been switched to OFF
position.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 3 –Fuel Indications

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

PRESSURE REFUELING PANEL

Refueling Station
The Fueling Valve (3)
Consist the following components for monitoring and
•is an electrical controlled and hydraulically operated valve
controlling for refueling / defueling and fueling transfer
•has a fueling solenoid with a manual override button
− Refuel Panel
− opens, when the fueling solenoid is energized and
− Fueling Receptacle
refuel pressure available
− Fueling Power Control Switch
− closes, when the fueling solenoid is deenergized or no
−Fueling Valves
refuel pressure available
The Test Gages & Aux Fueling Power Control Switch
is normal in OFF position has two spring loaded positions
The Fueling Power Control Switch
− Test Gages
• supplies the refuel station with 28 V DC and 115 V AC
− Used to test fuel quantity system
Power, when the access door is in open position
− Aux Fueling Power Control
− Energizes fueling system if fueling power control switch
fails to activate system when door is opened
The Fueling Valve Position Light (blue) (3)
•indicates the energizing of the fueling valve solenoid
ON − Fueling valve solenoid is energized
OFF − Fueling valve solenoid is deenergized
is a push to test light
The Fueling Valve Switch (3)
• controls the voltage supply for the fueling valve solenoid
OPEN − 28 V DC for the fueling valve solenoid available
CLOSE − Fueling valve solenoid deenergized
The Fueling Quantity Indicator (3)
indicates the actual fuel quantity of the respective tank
receives the fuel quantity signal from the fuel indicator in the
cockpit displays faults in the fuel quantity indication system
by error codes A, F and 8 is interchangeable with the fuel
quantity indicator in the cockpit

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 4 –Fuel Panel

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

28−10 FUEL STORAGE

FUEL TANKS DESCRIPTION
Access panels (man size openings) on the wing lower
Integral fuel tanks, No. 1, No. 2 and center, store fuel surface provide access to each tank. Access openings
required to run the engines and auxiliary power unit. A surge in the tank ribs are provided for areas which are not
tank, outboard of each wing tank, collects fuel overflow and directly accessible through the access panels.
includes equipment for venting the fuel tanks to overboard. An access panel in the lower left surface of cavity No. 1
The main tank No. 1 and main tank No. 2 are located in the provides access to the center tank. Access to this panel
interspar area of each wing. The center tank is contained is through the left air condition bay.
entirely within the fuselage. The surge tanks are extensions A surge tank contains any fuel overflow from the tanks
of the main tanks. and provides the venting. A drain line allows the fuel
overflow to drain back into the center tank. A vent scoop
Primary wing structure is used for the airplane fuel tanks. is installed on the bottom skin of the surge tank. The
The tanks are located between the front and rear spars and vent scoop provides a venting of the surge tank and a
between the upper and lower wing skin. Solid ”tank end” ribs positive tank pressure during flight. In case of over
closes the ends of each tank, while all other wing ribs act as fuelling, the fuel will flow through the vent scoop out of
fuel baffles to reduce fuel slosh. The center tank is divided the surge tank on the ground.
into three cavities by spanwise beams. In the access panels 1, 4, 6, 8 and 10 of tank No. 1 and
All fuel tanks are fuel thigh. Close metal−to−metal fit of all No. 2 are drip sticks installed for an alternate fuel
parts forms the basic seal. Sealing compounds and sealed quantity measuring.
fasteners are used on all joints to complete the fluid tight
seal. Two of the wing ribs contain a series of baffle check
valves to prevent fuel swapping within the tank and fuel flow
away from the boost pumps during flight.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 5 –Fuel Tank Arrangement

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

FUEL TANK ACCESS PANEL

Maintenance advice:
Access panels permit entry into the airplane fuel tanks
Fuel measuring sticks are installed in the fuel tank access panels
for inspection or component repair.
No.1, 4, 6, 8 and 10. ”Indexing Notches” are located at the
inboard section of the fuel tank access of the wing. The diameter
The access panels in positions No. 1 and No. 2 are
of fuel tank access panel No.10 is about 2 inches less than the
molded aluminum honeycomb. All other wing tank
other panels.
access panels are cast aluminum. The access panels
A ”Flame Arrester Pressure Relief Valve” is installed in the
are installed in the tank. The panels are installed by
access panel
inserting screws through a clamp ring on the outside,
flush with wing panel and threading the screws into No. 13. The pressure relief valve must be closed before panel
reinstallation.
captive nuts in the access panel. A molded rubber seal
ring, fitted into a grove in the panel seal face, supplies a
static type fluid seal between the access panel and the
wing skin. Fuel tank access panels are electrically
bonded to the wing structure by coating the gasket and
clamp ring with anticorrosion grease.
The center tank has one access panel, which is located
in the left air condition bay. Each surge tank has two
access panels. The No.1 and No. 2 tanks have 12
access panels each. In addition, two smaller panels in
each lower wing skin, permit access to the fuel boost
pumps.
Molded Rubber Seal
This type of seal is fixed to the access panel (old type).
This seal is replaceable on new type access panels only.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 6 –Fuel Tank Access Panels

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The Center Tank Access Panel

The center tank Access Panel consists of cast
aluminum and is located in the left air condition bay.
The panel is additional sealed because of the built in
position (air conditioning bay).

Installation Note: Do not use grease for screw

installation. Thighten each bolt with small turn. Then
tighten a different bolt with a small turn until you have
the correct torque value for each bolt. The run on
torque is the torque necessary to turn the bolt against
the friction of the threads. Ref. to Maintenance Manual

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 7 –Fuel Tank Access Panels

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BAFFLE CHECK VALVES

The baffle check valves prevent fuel flow in

the outboard direction but permit fuel flow
in the inboard direction. Thus the baffle
check valves prevent the boost pump inlets
from being uncovered and improve airplane
stability.

The valves are located on the bottom of

wing ribs at wing buttock line 157.0, 227.0
and 557.0.

The baffle check valves are free swinging

and flow actuated. Inboard fuel flow opens
and outboard flow closes the valves.

Figure 8 –Fuel Tank Baffle Valves

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

TANK DRAIN VALVES

The sump drain valves are used for draining accumulated

moisture from each tank and for draining trapped fuel
remaining after defueling.

In each tank is one drain valve at the lowest point

installed.

Description Main Tank Sump Drain Valves

The sump drain valves for the No. 1 and No. 2 tank are
spring-loaded closed, poppet-type valves flush-mounted in
the wing lower skin. A screen over each valve inlet
protects the unit from contaminants.

To drain the main tank fuel sump, the poppet is pushed up

to open the valve and let fuel drain through the drain hole
in the center of the valve to outboard.

Maintenance Practices
The primary drain O-Ring of the main tank drain valve
poppet, can be replaced without draining or entering the
fuel tank by placing the valve in the seal service position
( old valve version P/N VD82-2941 ). If it is a new valve
( P/N 87-5273 ) installed, a wrench tool is required in
order to replace the O-Rings by release the valve core
plug. The removal of the valve assembly requires draining
and entering the tank.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 9 –Fuel Tank Drain Valves

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CENTER TANK DRAIN VALVES

The center tank train valve is used for draining the Maintenance Practices/Sump Drain Valve (Tool-
accumulated moisture and trapped fuel remaining after Operated)
defueling. 1. Sump drain valve change
• The center tank must be defueled for drain valve
The center tank drain valve is installed in the wing lower removal.
skin in the center section. The access door is located • Access to the assembly is through lightening holes in the
between the air conditioning bays in front of the marker keel beam.
beacon antenna. • The bonding jumper must be disconnected from the
airplane structure.
The center tank sump drain valve is a spring-loaded • The retaining plate must be removed.
closed, flapper-type check valve. A sump drain assembly • The drain assembly can be turned clockwise until it
connects the valve upper housing with the lower body moves freely from the upper housing.
section of the airplane through the keel beam area. • Installation in a vice versa sequence

The sump is drained as follows: Maintenance Practices/Sump Drain Valve (Rod-

1. If the sump drain valve does not have an actuating rod Actuated)
(Tool-Operated) 1. Sump drain valve change
• To drain, insert a broad-bladed screwdriver in slot, turn The valve can be removed without defueling the center
clockwise and push up on the plunger. tank, because of a flapper assembly. It closes the valve
• To close, release the plunger, let the slot align fore and opening in the tank, when the drain valve has been
aft, and close access door. removed.
2. If the sump drain valve has an actuating rod (Rod- The removal and installation procedure is similar as the
Actuated) tool-operated one.

To drain, pull on the actuating rod.

To close, release the rod and close the access door.

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Figure 10 –Fuel C.T. Drain Valves

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APU SHROUD DRAIN

A shroud drain is provided to drain

moisture or any fuel leaking from the
APU fuel line.

The drain line is located in the left main

wheel well.

The shroud drain consists of a drain line

connected to a drain manifold. The
drain manifold contains a flame arrestor
and a drain cock. The line terminates in
a drain mast.

During operation, only moisture drains

from the mast. If the APU fuel line is
leaking, fuel drains from the drain mast.
The drain cock is used to check for and
drain off any leaking fuel while on the
ground. The drain cock must be closed
for flight.

NOTE: It is optional to have the

hydraulic reservoir vent line connected
to the drain mast on new B737
airplanes (see ATA 29 Hydraulic). Figure 11 –APU Shroud Drain

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

28-20 FUEL VENT SYSTEM A fuel float valve opens and closes corresponding to the fuel
TANK VENT SYSTEM level in the tanks and airplane attitude changes. As the fuel
level rises the valves begin to close. As the fuel level lowers,
The fuel vent system prevents damage to the tank structure by the valve begins to open. A float stop supports the float in its
providing positive venting of all fuel tanks, regardless of full open position.
airplane attitude. During flight, the system also helps to The fuel vent scoop consists of a recessed ram air scoop
decrease fuel evaporation and assists the fuel boost pumps by mounted in a lower wing surface access panel, and a five-
providing a small positive pressure head on the fuel. inch standpipe assembly. A screen is installed at the top of
The components associated with the tank vent system are the standpipe to prevent the entrance of foreign matter. The
located inside the fuel tanks. Vent scoops are located on the design of the scoop is such that icing conditions have
bottom skin of surge tanks. minimum effect on ram air pressure.
Two parallel vent channels extend into the main and center All fuel tanks are fuel tight. Close metal-to-metal fit of all parts
tanks from the surge tanks. Two stringers attached to the wing forms the basic seal. Sealing compounds and sealed
upper skin form each hat section channel. The channels open fasteners are used on all joints to complete the fluid tight seal.
into each surge tank. The aft channel terminates near the wing Two of the wing ribs contain a series of baffle check valves to
root in the wing tanks and is connected to a vent tube. The prevent fuel flow away from the boost pumps.
forward channel passes through the center tank and forms a Access to the vent components in the tanks is through the
continuous channel connecting the left and right surge tanks. fuel tank access panel.
The center tank is vented by tubing's and is connected to the During fueling of the tanks, if the fueling system fails to close
forward channel. the fueling valve, excess fuel enters the climb port. (Float
A tube is mounted to the channel in tanks No. 1 and 2. In the valve is closed.) Through the vent channel, the fuel flows to
center tank, tubing is mounted to the channel at two places. the surge tank and overflow through the vent scoop. Fuel
Two drain check valves are installed in the center tank vent trapped in the surge tank (maximum 114 Liters, 30 US
tubing, to drain the tubing/channel/surge tanks. gallons) flows to center tank, after initial burn off, through the
A surge tank, located at the end of each wing, collects any vent system.
overflow fuel passing through the vent channels. The overflow During climb, the climb port is exposed, float valve is closed,
fuel flows through a surge tank drain line into the center tank. allowing air/vapor to be vented through the vent scoop.
Tank No. 1 and 2 float valves are located in the channel of the During cruise, air enters the tank through the normal port
wing tanks. The center tank float valve is in the vent tubing. (float valve is open) providing slight pressurization.
During descent, air enters the tank through the normal port
since float valve is open and climb port submerged in fuel.
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Figure 12 –Fuel Tank ventilation

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VENT SCOOP AND FLAME ARRESTER FLAME ARRESTER PRESSURE RELIEF VALVE

Fuel Vent Scoop Surge Tank Flame Arrester Pressure Relief Valve
The fuel vent scoops, located in the outboard half of A positive and negative pressure relief valve is used to
the vent surge tanks, are the overboard vents for the bypass the fuel vent scoop for fuel system venting if the
vent system. Each scoop consists of a recessed surge tank flame arrester core cells are plugged such as
ram air scoop mounted in a lower wing surface from severe icing conditions. The relief valve is installed on
access panel and a five inch standpipe assembly. the access panel for the inboard half of the surge tank.
The diverging walls of the standpipe act as a plenum A spring loaded poppet, on the relief valve, with outer
chamber ensuring that surface flush with the wing lower skin is triggered open
ram air will be applied to the vent system during all through a latching mechanism by a pressure or vacuum
flight attitudes of the air- force acting on a diaphragm.
plane. The upper chamber of the diaphragm is vented to the
A stainless steel, cellular core flame arrester is atmosphere through three ambient air holes in the lower
installed at the top of the outer surface of the valve while the lower chamber is vented
standpipe to suppress flame transmission into the to the interior of the surge tank.
tanks and vent system from an external ignition When the poppet opens, it is raised about 1 inch above the
source. The flame arrester also serves as a debris wing lower skin surface which causes a manual reset knob
screen which may be subject to airflow restriction and stem to protrude about the same distance downward
caused by debris or severe icing conditions. out of the center of the poppet.
A surge tank flame arrester pressure relief valve is Once actuated open, the poppet must be manually reset to
used to provide the necessary venting when airflow the closed position at the conclusion of the flight. Manual
through the flame arrester is restricted. reset is accomplished by pulling reset knob downward until
the poppet is re-latched in the closed position. The relief
valve is designed to open at 2.0 to 2.7 psi increasing
positive pressure and -4 to -10 inches of water increasing
negative pressure or vacuum.

Maintenance advice:
If the flame arrester pressure relief valve was found open,
you must check the flame arrester for condition (clogging).

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Figure 13 –Fuel Tank Arrester

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Figure 14 –Fuel Tank Pressure Relief VLV

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28-20 PRESSURE FUELING SYSTEM

PRESSURE FUELING STATION

The pressure fueling panel is used for filling the tanks,

defueling the tanks or fuel transfer between the tanks.
The pressure fueling panel (P15) is located in the fueling
station.
The fueling panel consists of 3 fuel quantity indicators, 3
fueling valve switches, 3 blue valve position lights, switch for
indicator test or for auxiliary fueling power control.
Access to the fueling panel is by opening the fueling station
door.
Power
The power required for the fueling operation is 28 volt DC and
115 volt AC obtained from ground power, APU or battery.
Control
When the fueling station door is opened, the fueling power
switch is actuated and the panel is illuminated by 3 white
lights. (If the fueling power switch fails to operate, power to the
panel can be supplied by holding the auxiliary fueling power
control switch.)
Operation
Prior to the fueling operation, the 3 blue lights and quantity
indicators are checked for proper operation. Using the fueling
valve switches in the open position, allows the fueling valve to
open, blue lights illuminate, and fuel proceeds to the
respective tank. Fueling can be stopped by placing the valve
switch to the closed position, or fueling stops automatically by
a float switch when the tank is full. Upon completion of fueling,
closing of the door removes power from the fueling station.

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Figure 15 – Refuel Station

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FUELING OPERATION
Operation
The fueling system provides a means of filling the fuel With the fueling hose connected to the fueling manifold and
tanks on the airplane. the three fueling valves open, fuel is supplied to the three
tanks.
Fueling can be accomplished by the use of a single Supply to the main tank No. 1 passes through main tank No.
pressure fueling station through which all tanks can be 2 and center tank. Supply to the center tank passes through
filled partially or completely. Alternately, the main tanks main tank No. 2 and a check valve. Upstream of the check
can be filled through the over wing ports. The center tank valve are vent lines from the two center tank pumps.
can be filled by transferring fuel from the main tanks using These lines ensure automatic repriming of the pumps during
the fueling station. fueling. Supply to the main tank No. 2 is through a restrictor
to ensure same fueling rate as for main tank No. 1.
The components associated with the fueling system are During the fueling operation, if the fueling valves fail to close,
the fueling station located under the right wing leading the excess fuel enters the vent system and proceeds
edge outboard of the engine, 3 float switches one inside overboard through the vent surge tanks and the vent scoop.
each fuel tank and 2 over wing ports on top of each main
tank. Backup Operation
The main fuel tanks can be filled through the over wing ports.
All tanks can be filled from the fueling station. The tanks The center tank can be filled by transferring fuel from the
can be filled to any desired quantity by using the control main tanks.
panel. The float switches sense full tank condition and
automatically stop the fueling. Power requirements are
supplied by ground power, APU or the battery. Pressure
fueling can also be done manually. Maximum fuel
pressure is 55 psi.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 16 – Refuel Station Receptacle

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FUELING MANIFOLD

The fueling manifold is used as a distribution center for the Maintenance Practices
pressure fueling, defueling and fuel transfer. The fueling manifold and its components can be removed and
The fueling manifold is located on the right wing front spar in replaced without defueling the tanks. Removal of the manifold
the fueling station. requires evacuating fuel from the manifold by depressing the
The manifold consists of a fueling receptacle with a poppet poppet valve in the receptacle, disconnecting electrical
valve, three solenoid controlled fuel pressure operated connectors to the fueling valves, removing the fueling valves
valves with manual overrides, and four outlet ports, three of and bolts between the manifold and check valves. Removal
which contain check valves. and replacement of the fueling check valves requires removal
The access to the manifold is by opening the fueling station of the manifold. The fueling hose adapter can be removed
door. and replaced without removing the manifold.
With the fueling hose connected, a plunger in the hose
nozzle opens the poppet valve allowing the fuel to enter the
fueling manifold. Fuel through the orifice enters the chamber
above the diaphragm, thus equalizing the pressure across
the diaphragm and the spring keeps the valve closed. When
the solenoid is energized or the manual override plunger
pushed and held, fuel pressure above the diaphragm is
relieved and fueling valve opens. Fuel from the manifold
proceeds to the respective tank through the check valve.
When the solenoid is de-energized or the manual override is
released, pressure is equalized across the diaphragm and
the fueling valve closes.
During defueling operation, with the defueling valve open,
fuel enters the fueling manifold and is taken out by a hose
through the fueling receptacle.
During transfer operation, with the defueling valve open and
the tank to be filled fueling valve open, fuel proceeds from
the fueling manifold to the tank.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 17 – Refuel Station Manifold

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FLOAT SWITCH

The float switches, one for each tank,

prevent the tanks from being
overfilled.

The switches are located near the top

of each tank and sense full tank
quantity.
The switch consists of a capsule
surrounded by a cork float containing
a magnet; the whole assembly is in a
cylindrical container.
Access to the switches is through a
fuel tank access panel.
Power supply is 28 volt DC from the
fueling station.

During pressure fueling with the tank

less than full, the float switch allows
power to the fueling valve which will
then open with fuel pressure. When
the tank is full, the float switch
interrupts power to the fueling valve
causing it to close.

Figure 18 – Float Switch

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

FUELING CIRCUIT

If the battery switch is ON, battery

provides power to relay R11. If ground
power is used, the Bus Protection Panel
TR unit provides power for relay R11.
When the fueling bay door is open, the
fueling power switch (or holding the
auxiliary fueling power control switch)
provides a ground for relay R11. This will
provide 28 volts DC for fueling bay
illumination, press-to-test of valve
position lights and valve position
switches.

Placing the valve position switch to

OPEN energizes the valve solenoid
through the closed float switch; the blue
light is illuminated when solenoid is
energized. When the tank is full, the float
switch opens, de-energizing the solenoid,
valve closes and blue light is
extinguished.
Fueling can be stopped at any time by
placing the valve position switch to Figure 19 – Fueling Circuit
CLOSE.

Backup Operation
If DC power is not available, the fueling
valve can be opened by the manual
override button next to the solenoid.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 20 – Fueling Electric Circuit

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OVERWING FUELING PORT

The fuel tanks No. 1 and No. 2 can be filled by

gravity through the over wing fueling ports.

The ports are located in the top of the wing towards

the wing.

Each assembly consists of a flush fitting cap with a

locking mechanism and a retainer lanyard. The filler
cap is opened by rotating the cap counterclockwise.
When the filler cap is opened, the main tank can be
filled by gravity. The quantity of fuel that can be put
into the tank through the over wing port. The fuel
quantity is considerably less than by pressure
fueling, because of port location.

CAUTION: Do not open over wing filler cap when

the tank has been full refueled by pressure fueling.
More than 400 KG fuel spills out over the wing.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 21 – Overwing Fill Port

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28-40 FUEL FEED SYSTEM

ENGINE FUEL FEED SYSTEM
The distribution system allows fuel to be supplied to the
engines and APU. In addition, the system can be used for
defueling the tanks and for fuel transfer between the tanks.
The distribution system utilizes pumps, valves and tubing
for engine and APU feed. The fueling station is used for
defueling and transfer operations which are only possible
on the ground.
The fuel distribution components are located inside the
main tanks and on the front and rear spars of the main
tanks.
General Operation
A. Engine Feed
- Fuel is first supplied to both engines from the center tank
and then from the respective tanks to engines. The
crossfeed valve allows fuel from one tank to be supplied to
both engines. Fuel can also be supplied from main tanks by
suction to both engines.
B. APU Feed
- Fuel to the APU is primarily supplied from tank No. 1, but
it can be supplied from any tank using the engine feed
components.
C. Defueling
- All tanks can be defueled through the fueling manifold
using engine feed components.
D. Transfer
- Fuel can be transferred between the tanks using engine
feed components and the fueling manifold.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 22– Fueling Feed System

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

ENGINE FUEL FEED SYSTEM

Fuel Transfer
Fuel can be transferred between the tanks on the ground
only, because the fueling system is utilized for this
operation. In all cases of fuel transfer the defueling valve
must be open (in some cases, the crossfeed valve must
be open also). For example, transfer of fuel from No. 1 to
No. 2 tank involves:
- Boost pumps in No. 1 tank operating
- Crossfeed valve open
- Defueling valve open
- No. 2 tank fueling valve open
Fuel from No. 1 tank is pumped by the boost pumps, via
the crossfeed manifold, to No. 2 tank. Similar operations
are required for other tank-to-tank transfer.

Defueling
In all cases of defueling, the defueling valve must be open
(in some cases, the crossfeed valve must be open also). A
fuel hose is attached to the fueling receptacle. For
example, defueling No. 1 tank (using pumps) involves:
- Boost pumps in No. 1 tank operating.
- Crossfeed valve open.
- Defueling valve open.
Fuel from No. 1 tank flows through the pumps, manifold,
valves and fueling hose to storage. Similar operations are
required for defueling the other tanks.
Suction defueling can be used, but it is not selective.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 23– Fueling Feed System

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FUEL BOOST PUMPS

The pump removal and drain valves are used during the
MAIN AND CENTER TANK BOOST PUMPS replacement of the pump. When the removal valve handle is in the
The six fuel boost pumps, two per tank, deliver fuel under pressure closed position it will interfere with the access panel preventing its
from various tanks to the two engines. The pumps are also used for reinstallation.
defueling the tanks and for fuel transfer between the tanks. The The jet pump for the center tank fuel scavenge takes residual fuel
center tank scavenge system utilizes forward left pump discharge from the center tank and delivers it to tank No 1.
flow for jet pump operation. The jet pumps for the main tank water scavenge take fuel/water
The main tank water scavenge system utilizes forward main tank mixture from the low points in the main tanks and deliver it to the
pumps discharge flow for jet pump operation. main tank pump inlets.
The main tank forward pumps are installed on the front spars. The Maintenance Practices
main tank aft and center tank pumps are located in dry bays in the Removal and installation of all pumps is the same and can be
main tanks. performed without defueling the tanks. The pump removal valve is
The boost pump consists of a three-phase motor-driven centrifugal closed, fuel drained from the pump and two lower mounting bolts
impeller, inlet port with pump removal valve and handle, drain cock, on the clamp are removed. The clamp can be swung out and pump
vent port, discharge port with connection to a pressure switch and a removed. Prior to installation of the pump, approximately 1/2 pint of
check valve. The pump is held in position by two clamps. On the fuel is poured into the pump for repriming.
forward pumps, part of the discharge flow is directed to jet pumps. The jet pump nozzle can be removed without defueling the tank.
NOTE: MAIN TANK AND CENTER TANK PUMPS ARE
IDENTICAL PUMP OUTLET CHECK VALVES ARE DIFFERENT CAUTION: WHEN REMOVING A CENTER TANK BOOST PUMP,
FOR PUMPS IN MAIN AND CENTER TANK. (VALVES ARE ALSO CLOSE THE REMOVAL VALVE FOR OPPOSITE THE
INDEXED ON MANIFOLD SIDE TO PREVENT MISMATCHING.) CENTER TANK BOOST PUMP TO PREVENT FUEL FLOWING
Access BACK THROUGH THE REPRIME LINE.
The forward boost pumps are accessible by extending the leading
edge flaps. The aft and center pumps are accessible through panels APU DC BOOST PUMP
on the bottom wing skin. The APU DC boost pump supplies fuel under pressure from tank
Power No. 1 to the APU fuel feed line as necessary for cold starting or
The pump use 115 volt AC, 3Ø, 400 Hz power. With the motor after air has entered the fuel feed line.
operating, fuel enters the pump and is delivered to the manifold The boost pump is mounted on the left lower inboard trailing edge
through the check valve. Upstream of the check valve, the pump panel and pumps fuel through tube lines which penetrate the left
performance is monitored by a pressure switch. Part of the incoming wing rear spar. The pump operates on 28 Volt DC Power and will
fuel flow (approximately 2 gallons per minute) is used for cooling run for approximately 1 minute during APU starting.
and lubricating the pump and is discharged into the tank through the
vent connection.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 24– Fuel Booster Pumps

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MAIN AND CENTER TANK BOOST PUMP

POWER CONTROL
The boost pump power sources are arranged to
ensure that in the event of a single generator failure,
at least one pump in each tank will remain powered.
The circuit breakers and relays are located in the P6
panel. The boost pump switches are located on the
P5-2 panel.

Power
The power required is 28 Volt DC for the relay and
115 Volt AC for the pump.

Operation
Placing the pump switch to the ON position energizes
the boost pump relay which in turn connects 115 Volt
AC to the pump.
The main tank aft pumps are powered from the
transfer busses so that at low fuel level, if a generator
is lost, both aft pumps remain powered. The
remaining pumps are powered from main busses.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 25– Fuel Booster Pump Circuit

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BOOST PUMP CHECK VALVES

The fuel boost pump check valve closes the

boost pump outlet line when the pressure drops
below a predetermined amount, thus preventing
reverse fuel flow to the boost pump.
The check valves are located on the pump
discharge lines inside the fuel tanks.
The valve consists of a spring-loaded-closed
flapper valve mounted in a housing. The housing
has three ports: fuel in, fuel out and a pressure
sensing port. The pressure sensing port is
located upstream of the flapper valve. Main tank
boost pump check valves differ from center tank
boost pump check valves as the cracking
pressure for main tank boost pump check valves
is higher than the cracking pressure for center
tank valves.
- cracking pressure of the center tank check
valve is 1,5 psi,
- cracking pressure of the wing tank check
valve is 12 psi.
The access to the valves is by entering the fuel
tanks.

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Figure 26– Fuel Booster Pump Check VLV’s

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ENGINE FUEL FEED VALVES Operation

ENGINE FUEL SHUTOFF VALVE & ENGINE When the motor is energized, the actuator assembly drives
the gate to close or open the valve. The fuel shutoff valve is
CROSSFEED VALVE
opened by movement of the engine start lever from CUTOFF
Fuel to each engine passes through the engine’s to IDLE position. It is closed by the start lever or by the
respective fuel shutoff valve. For supplying fuel from one operation of the corresponding fire handle.
tank to both engines, the crossfeed valve is used. The crossfeed valve is opened or closed by a rotary switch
located on the P5-2 panel. The valve can also be operated by
The two fuel shutoff valves are located on the front spars the manual override handle providing that the corresponding
outboard of the engines. The crossfeed valve is located on circuit breaker has been pulled. The override handle also
the rear spar on the right side. indicates the position of the valve.
The thermal relief valves limit the pressure trapped in the
The valve is motor operated, sliding gate, semi-submerged manifold to 45-60 psi with the system shut down.
with manual override handle. The valve consists of gate, APU Fuel Shutoff Valve
actuator, and port adapter assemblies. The port adapter The APU fuel valve is mounted on the left wing rear spar and
assembly is mounted inside the tank and supports the controls delivery of fuel from tank No. 1 to the APU.
manifold. The gate assembly slides into the adapter from The valve is an electric motor-driven rotary gate valve with
the dry side, and the actuator couples to the gate shaft with manual override handle. The manual override handle
an index-tooth spline. Three thermal relief valves are provides a visual check of the valve position and a manual
provided. method of positioning the valve when the electric motor is not
Access energized.
The fuel shutoff valves are accessible through individual To limit pressure buildup in the fuel lines, the valve housing
panels located out board of the engines on the wing incorporates a thermal relief valve.
leading edge. The crossfeed valve is accessible from the The valve is operated by 28 Volt DC Power and is controlled
right main wheel well. by the APU master switch for opening and closing, or by APU
Power fire handle for closing only.
The valves require 28 Volt DC power.
. NOTE: The main tank or tanks must be defueled to the
quantity indicated on the placard prior to removal of the valve.

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Figure 27 – Engine Feed VLV’s

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ENGINE FUEL SHUTOFF VALVE CROSSFEED VALVE

ELECTRICAL CIRCUIT ELECTRICAL CIRCUIT
Fuel from the tanks passes through the two fuel shutoff valves A fuel crossfeed valve provides the means of directing fuel to
before entering the engine fuel systems.
both engines from a single tank.
The valves are located on the front spars outboard of the The valve is located on the rear spar of tank No. 2, the
engines. The indicating lights and associated circuits are on indicating light, switch and associated circuits are in the P5-2
the P5 panel. Controls for valve operation are below the start panel.
levers and on P8 fire control panel.

Power The switch for valve operation is a rotary type. Above the
The power for the light and valve operation is from 28 volt DC switch is a blue crossfeed VALVE OPEN light.
HOT BATTERY BUS. The circuit breakers are on P6 panel. Power
The power for the valve and light operation is from 28 Volt DC
Operation BATTERY BUS the circuit breaker is on the P6 panel.
The valve is opened and closed by the engine start lever, and Operation
it can be closed by the respective fire handle. The indicating The valve is opened and closed by a rotary switch on P5-2
FUEL VALVE CLOSED blue light operates as follows: panel. The indicating VALVE OPEN blue light operates as
- Valve closed - dim blue light follows:
- Valve in transit - bright blue light - Valve closed - no light
- Valve open - no light - Valve in transit - bright blue light
Power for the light is available when relay R41 is energized by - Valve open - dim blue light
battery switch ON. The light is illuminated when a ground is Light is illuminated when ground available through both
available through both transistors (bright) or through a zener transistors (bright) or through a zener diode and transistor
diode and transistor (dim). (dim).

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Figure 28 – Engine Feed VLV’s Eclectics Circuit

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ENGINE FUEL SHUTOFF VALVE SWITCH

The switch for the fuel shutoff valve is installed on

brackets in pilot’s control stand. There is one switch for
each start lever.
You can get access to the switch for the fuel shutoff
valve for the No. 1 engine from the left side of the pilot’s
control stand.
You can get access to the switch for the fuel shutoff
valve for the No. 2 engine from the right side of the
pilot’s control stand.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 29 – Engine Fuel Shutoff Valve Switch

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DEFUELING MANUAL DEFUELING VALVE

The fuel system is designed to permit complete or partial The defueling valve is used for defueling of the tanks and
defueling of one or more tanks, or to permit fuel transfer to for fuel transfer between the tanks.
other tanks while the airplane is on ground. To do the defueling
or fuel transfer, parts of the pressure fueling, engine fuel feed, The valve is located inboard of the fueling station.
and defueling systems are used in combination.
There are four different procedures in order to defuel the fuel The valve is a manually operated, sliding gate, semi-
tanks submerged unit. The valve consists of a gate, actuator,
1. Defuel through the pressure fueling receptacle with the and port adapter assemblies. The port adapter assembly
airplane boost pumps and the defueling (suction) pump on the is mounted inside the tank and supports the fuel line. The
refuel truck. This procedure defuels all of the tanks at the gate assembly slides into the adapter from the dry side.
same time. It is the fastest procedure to defuel the tanks. Access to the valve is through a panel inboard of the
2. Defuel through the pressure fueling receptacle with the fueling station.
airplane boost pumps only.
Operation
3. Defuel through the pressure fueling receptacle with the The valve is opened and closed by the use of the red
defueling (suction) pump on the fuel truck only. Use this valve handle. When the valve is open, the access panel
procedure only if it is necessary. It is a very slow procedure. can not be closed due to panel/handle interference .
4. Defuel through the pressure fueling receptacle with the
external power control unit. For this procedure, it is a special
ground support equipment required.

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Figure 30 – Manual Defueling Valve

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28-50 CENTER TANK FUEL SCAVENGE SYSTEM

CENTER TANK FUEL SCAVENGE FLOW INDICATOR
The scavenge fuel shutoff valve, when energized, allows fuel (OPTIONAL)
from the No. 1 forward boost pump to the center wing
scavenge jet pump. The scavenge flow indicator is used for the center tank fuel
scavenge system.
The valve is mounted on the front spar of the No. 1 fuel tank
outboard of the No. 1 forward boost pump. The indicator is located on the front spar of the No. 1 fuel
tank.
The unit consists of a valve body and solenoid assembly It consists of a flapper check valve assembly which is
containing a solenoid, plunger, poppet, pins and spring. connected to a pointer on the indicator. The deflection of the
The valve is accessible from the front spar of the No. 1 tank. flapper is proportional to the flow through the system.
The indicator has three color bands:
Operation - Blue band - no flow
When the center tank boost pump switches are on, the - Red band - flow up to 280 lb / hr at 21°C (70°F)
solenoid is relaxed and the plunger/poppet assembly is spring- - Green band - flow above 280 lb / hr
loaded closed against the valve seat insert.
Turning both center tank boost pump switches off will energize Access to the indicator is by extending the leading edge flap.
the solenoid. The poppet will pull away from the valve seat Operation
allowing fuel flaw from the No. 1 tank forward boost pump to The system is checked by observing the indicator with the
the scavenge fuel jet pump. center tank boost pumps switched off and the tank No. 1
forward boost pump switched on.
Maintenance Practices Maintenance Practices
Removal of the valve requires defueling the No. 1 tank and The indicator removal requires defueling of tank No. 1.
extension of the leading edge flaps to gain access to the
attaching hardware. At the front spar, disconnect the electrical
connector and bonding jumper from the valve. Remove the nut
and washer. Inside the No. 1 tank disconnect the fuel lines and
remove the valve.

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ATA 28 – Fuel ATA 104 - Level 3 B1/B2

Figure 31– Fuel Scavenge System

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CONTROL LOGIC

The fuel scavenge circuit controls the operation of the

scavenge fuel shutoff valve.

The circuit components are located in the P5 and P6

panels and in the electronic equipment compartment.

The scavenge circuit consists of power sources, two

center tank boost pump switches and relays, M528
Engine Accessory Module and the scavenge fuel shutoff
valve solenoid.

Power
Power supply for the center tank boost pumps is 115 Volt
AC and for the control circuit 28 Volt DC.

Operation
The circuit is designed to energize the scavenge fuel
shutoff valve solenoid whenever both center tank boost
pump switches are positioned to OFF and keep the
solenoid energized for 20 minutes.
A prerequisite to this operation is that one (or both) center
tank boost switches, must be turned ON to arm the M528
control logic. If this is not accomplished, the scavenge fuel
shutoff valve solenoid will not be energized even through
both center tank boost pump switches are in the off
position.

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Figure 32– Fuel Scavenge System Control Logic

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FUEL SCAVENGE JET PUMP

The fuel scavenge jet pump causes a low pressure in

the fuel scavenge transfer line when the forward fuel
boost pump of tank No. 1 is running and the electrically
controlled fuel scavenge shutoff valve is energized.
Inspection / Check
It is not necessary to go into or defuel the tank in order
to clean the nozzle opening on the jet pump. The
flapper valve prevents fuel flow when the nozzle has
been removed.

Figure 33– Fuel Scavenge Jet Pump

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28- 60 FUEL QUANTITY INDICATING

SYSTEM Compensator Units are hollow coaxial cylindrical
The fuel quantity indicating units measure the weight capacitors mounted vertically to ribs inside each fuel tank at
of usable fuel in the tanks. Two types of quantity the lowest portion so that the compensator is always
indicators are used, capacitance for indicators and submerged in fuel. The compensator is connected
manual measuring sticks. The measuring sticks are electrically into the gagging circuit to compensate for the
provided for the main tanks only. different dielectric constants of fuels of differing chemical
The capacitance gagging system tank units and compositions.
compensators are located inside the tanks. The The tank units are connected together in parallel. The wiring
measuring sticks are located in main tank access harness from each main tank is connected to a bussing plug
panels. Main Tank - 12 tank units, 1 compensator, 5 located in the front spar of the wing. The center tank bussing
measuring sticks. plug is located on the forward wall of the left main wheel
Center Tank - 2 tank units, 1 compensator. well.
The TSDs (Transient Suppression Devices) suppress
Tank Units are hollow coaxial cylindrical capacitors electrical transients,
mounted vertically to ribs inside each fuel tank. Fuel preventing them from reaching the FQIS wiring and
is used as the dielectric. The inner tube is profiled to components inside the fuel tanks.
correspond to the irregular shape of the inside of the Drip Sticks are calibrated tubes which can be pulled down
fuel tank. The tank units are located throughout the for reading of fuel height.
tanks so an accurate reading is provided whatever The Indicators, Summation Unit and Trimmers are in the
the attitude of the airplane. flight
compartment. Repeater indicators are in the fueling station.

POWER
The indicators utilize 115 volt AC power.

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Figure 34– Fuel Quantity Indicating System

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FUEL QUANTITY INDICATION CIRCUIT

Fuel Summation Unit
The fuel quantity circuit and indicators measure the weight of The fuel summation unit takes individual fuel quantity indicator voltage
usable fuel in the fuel tanks. signals and sums them for an output voltage signal representative of the
System components are located in the flight compartment, fuel total fuel remaining. The flight management computer (FMC) system uses
this voltage signal to derive airplane gross weight related performance
tanks and at the fueling station. Flight compartment
data and provides display of total fuel.
components consist of fuel quantity indicators, capacitance Trim Adjustment Screws (Trimmer)
trimmer, summation unit and an indicator test switch. Fuel tank Capacitance trimmers, can be adjusted to standardize the total tank
components consist of tank units, compensator units and capacitance. This adjustment allows for replacement of indicators without
measuring sticks. Fueling quantity indicators and indicator test recalibrating the indicator gage.
switch are located at the P15 fueling station panel. Transient Suppression Device
The Transient Suppression Device is near the penetration to each fuel
tank. This is a safety device to prevent excess power from entering the
Fuel Quantity Indicator
fuel tank.
Digital fuel quantity indicators on P2 panel show the weight of Power
fuel in each tank. Each indicator consists of a microcomputer The power supply for the indicators is 115 volt AC from the STANDBY
system, radial and a digital liquid crystal display (LCD). The BUS or EXTERNAL POWER. If external power is plugged in and the
microcomputer measures the capacitance and resistive current power is good, relay R10 is energized connecting the fuel quantity circuit
in the tank units. It monitors the leakage current and when it to the external power AC BUS. Now, if AC power is applied to the airplane
reaches an unacceptable level, an error code 0 through 9 are such that the number 2 BUS transfer relay (R4) is energized, R 309 will
energize switching the fuel quantity circuit power source to the STANDBY
displayed, or the display blanks, to assist in troubleshooting
AC BUS. This is done to connect the fuel quantity circuit to the same
the system. power source as the flight management computer (FMC) to phase match
the summation unit total fuel quantity, fuel quantity reference, and ground
Fueling quantity indicators on the P15 panel show the weight reference analog voltages with the FMC reference voltage.
of fuel in each tank. The fueling quantity indicators function as Operation
repeaters of the fuel quantity indicators located in the flight The tank units function as variable capacitors, their capacitance value
compartment and aid the ground crew in fueling or defueling being determined by fuel level in the tank.
A single chip microcomputer system within the indicator performs the
operations. The fueling quantity indicators are interchangeable
measurement of the current through the tank units. The microcomputer
with the fuel quantity indicators. The fueling quantity indicators converts the capacitance current to a radial and a digital readout of fuel
are operative only when the fueling station door is open. Error quantity. The microcomputer monitors the fuel quantity indicating system
codes 8, A and F are displayed or the display cycles ON/OFF and provides an error indication and code to assist in troubleshooting.
or blanks on the repeater indicator.

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Figure 35– Fuel Quantity Indicating System

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FUEL QUANTITY SYSTEM BUSSING PLUG AND

TRANSIENT SUPPRESSION DEVICE

Bussing Plug
Tank unit leads in a given tank, form a harness which
terminates at a receptacle. The receptacles for the No. 1
and No. 2 tanks are found in the applicable wing front
spar, and for the center tank in the left forward bulkhead
in the wheel well for the main landing gear. Attached to
each receptacle is a bussing plug assembly. The plug
assembly connects the tank units in parallel.

Transient Suppression Device

The TSD is a safety device that provides isolation of
excess power from the fuel quantity signals into the fuel
tank. A special EMI shielded cable connects the TSD to
the fuel tank bussing plug. The TSDs for the No. 1 and
No. 2 tanks are located on the respective wing front spar,
and for the center tank on the main wheel well ceiling
near the System A hydraulic reservoir.

Maintenance Advice:
For troubleshooting the Fuel Quantity Indicating System
a system tester can be connected to the indicator plug
and to the bussing plug. But the respective TSD must be
bypassed by using the TSD Bypass Cable.
For instruction see the AMM.

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Figure 36– Bussing Plug And Transient Suppression Device

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FUEL QUANTITY INDICATOR

Fuel Quantity Indicator

Digital fuel quantity indicators show the weight of fuel and
percentage of full capacity in each fuel tank.
Each fuel quantity indicator has a single chip microcomputer system
and a digital liquid crystal display (LCD). The microcomputer sends
periodically a short time AC signal of 10 Volt and 5 kHz over the
LO-Z wire directly to the tank units, there after to the compensator
unit and measures the current in the HI-Z wire. By this capacity
measuring method, the micro computer calculates the fuel quantity.
When a parameter reaches an unacceptable level, the fuel quantity
indicator shows either normal or zero fuel quantity, the word ERR,
and an error code between 0 and 9. The error code can be used to
help in troubleshooting the system. To clear any stored error codes,
including those stored by activating the self test switches, cycle
indicator power from ON to OFF to ON again, with the EP GND
FUELING and fuel QTY circuit breakers on the P6 panel.

Refuel Quantity Indicator

Refuel quantity indicators on the P15 panel show the weight of fuel
in the fuel tanks. The refuel quantity indicators function as repeaters
of the fuel quantity indicators found in the control cabin and help the Figure 37 - Fuel Quantity Indicator
ground crew in refuel or de-fueling operations.
The refuel quantity indicators are interchangeable with the fuel
quantity indicators. The refuel quantity indicators operate only when
the access door for the station is open.
When the access door is closed, power to the refuel quantity
indicators is interrupted by a reed-switch. The fueling quantity
indicators monitor the signal received from the fuel quantity
indicators and display an error code if a system malfunction exists.

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Figure 38 - Fuel Quantity Indicator Test

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FUEL QUANTITY INDICATOR TEST

By pressing of the fuel quantity test switch on pilots center Operation - Fueling Quantity Indicator
panel, a test of the fuel quantity indicators will be started. By switching of the Test Gages & Aux Fueling Power Control
Test sequence: Switch to ”Test Gages Position”, a test of the fueling indicator
- push and hold test switch until Indicators displays ”0” and will be started.
”ERR 4” - release test switch
- all segments ”ON” for 2 sec. Test sequence:
- all segments ”OFF” for 2 sec. - push and hold test switch to ”Test Gages” position
- if Error Codes memorized, each will displayed for 2 sec. - all segments ”ON”
one after another (numerical order). The highest code will - the indicators show ”Blank”
displayed at first. - maximum possible indicator value
- maximum possible indicator value of each indicator - release switch
- the indicators display actual quantity - the indicators display actual quantity
NOTE: The test is not possible or unusable, if the following
actual ERR- Codes are displayed: NOTE: During fueling quantity indicator test, the fuel quantity
2, 3, 5, 6 or 8 indicators in the cockpit display ERR-Code ”3”.
Does a ERR-Code ”3” exist, the indicator will display
ERR-Code ”2” while the test switch is being pressed.

When the following actual ERR-Codes are displayed:

1, 3, 5 or 7
the indicator accuracy will shift about 3 %.
During fueling indicator test, the repeater indicators on the
fueling station show a display test.

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Figure 39 - Fuel Quantity Indicator Test

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MEASURING STICK

The measuring sticks are used to measure the fuel quantity in General
tanks No. 1 and 2 by mechanical means. The fuel quantity You can refuel a tank with a fuel quantity indicating system
measurements obtained by measuring sticks assist the ground that does not operate correctly (a bad indicator). You can do
crew in fueling operations when electrical power for fuel this task with the fuel measuring sticks in the No. 1 tank or
quantity indicators is not available. The measuring sticks are the No. 2 tank. You must also use the applicable document to
also used for comparing the fuel weight indicated by the fuel calculate the fuel quantity from the values on the fuel
quantity indicators to the reading obtained by measuring measuring sticks. You must move fuel from a different tank if
sticks. you must refuel the center tank when its indicators do not
There are five measuring sticks for each main tank, located in operate.
wing fuel tank access panels No. 1, 4, 6, 8, and 10. You can also move fuel to refuel one of the wing tanks as an
alternative to the fuel measuring stick procedure. Use the
The measuring stick consists of a fiberglass tube in a housing
Fuel Measuring Sticks to Refuel the No. 1 Tank or the No. 2
attached to the access panel. The tube is graduated in inches, Tank when the Fuel Quantity Indicating System does not
gallons or kilograms, (a customer option). At the side of the operate
housing is a removal valve, which allows stick removal without
defueling.
NOTE: Calculating of the fuel weight by the Measuring Sticks
Operation (Ref. Maintenance Manual)
To determine the fuel quantity in the tank, the measuring stick
is unlocked and slowly lowered until the fuel enters the upper
end of the stick and drips out of the lower end. The drip hole is
marked on the base of the measuring stick with a red arrow.
Tables of capacities, corrected for airplane attitude, are found
in the measuring stick document.
Maintenance Practices
The sticks can be removed without defueling the tanks.
Removal of 3 retaining screws allows the stick assembly to be
pulled out from the access panel.

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Figure 40 - Fuel Measuring Stick

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The Open Type Fuel Measuring Stick (Drip stick) To find the fuel level
To operate the fuel measuring stick, apply a slight To find the fuel level, the fuel measuring stick is gently
pressure and turn the fuel measuring stick 90 degrees in raised until the magnetic latch is felt to engage. There is a
the counterclockwise direction. Slowly lower the fuel positive feel in this position since the magnetic latch is
measuring stick until fuel appears from the drip hole. Read strong and will jerk the fuel measuring stick into its position.
the fuel quantity on the fuel measuring stick to find the The fuel measuring stick is now read to find the fuel level in
quantity of fuel in the fuel tank. the fuel tank. If the fuel measuring stick is being monitored
during a refuel or defueling operation, the float and stick
The Magnetic Float Type Fuel Measuring Stick sub assembly will move with the fuel level. However, since
To operate the fuel measuring stick, it is unlocked from the the magnetic attachment between the magnets of the stick
base by applying a slight pressure and a 90 degree sub assembly and float causes a small friction against the
counterclockwise turn to the end of the stick. When the housing, the stick sub assembly and float will not move up
fuel measuring stick is unlocked, it is lowered to the fully smoothly with the fuel level, but in distinct jumps as the fuel
extended position. Depending on fuel level, a bearing level rises on the float to overcome the static friction. On
found on top of the stick will magnetically latch onto a declining fuel levels when defueling, friction will occur in the
magnet in the float before full extension. In this event, a opposite direction. Calibration marks on the stick sub
slight pressure should be applied to assure a full assembly are set to read on rising fuel levels and the most
extension. At the fully extended position, the fuel accurate reading will be immediately after a jump of the
measuring stick stops against a cushion giving a soft firm stick sub assembly.
feel.

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Figure 41 - Fuel Measuring Stick

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FUEL FEED LOW PRESSURE INDICATING SYSTEM

The condition of each boost pump is monitored by a Fuel Feed Low Pressure Switches
pressure switch, sensing pump output pressure, and an Fuel feed low pressure switches control the fuel low
amber LOW PRESSURE light operated by the pressure pressure indicating lights on the overhead panel. Each
switch. The low pressure warning system also actuates switch assembly consists of a cylindrical housing
the MASTER CAUTION and the FUEL Annunciator containing a diaphragm and a switch. Each switch has
lights. one inlet port for a pressure sensing line coming from a fuel
boost pump. The switch is vented to atmosphere. The fuel
The low pressure warning system circuits are located in feed low pressure switches are located on wing front spar.
the P5-2 panel.

The system consists of the six pressure switches, two

center tank pump switches, six LOW PRESSURE amber
lights, MASTER CAUTION and FUEL annunciate amber
lights.

The fuel feed low-pressure indicating system provides an

indication when the output pressure of any fuel boost
pump falls below a preset value. The system consists of
six pressure actuated switches and six indicator lights,
one for each fuel boost pump, together with connecting
fuel lines. If the outlet fuel pressure of a fuel boost pump
falls to the preset value, its fuel low pressure switch will
be actuated; this will illuminate the appropriate indicator
light on the overhead panel.

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Figure 42 - Fuel Feed Low Pressure Indicating System

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FUEL FEED LOW PRESSURE INDICATING SYSTEM

ELECTRIC SYSTEM
Low pressure fuel from any boost pump in any tank will
turn on a fuel feed low pressure light. Low pressure
from both fuel boost pumps in the same tank will turn on
the applicable low pressure lights and in addition, the
fuel annunciate light and the master caution lights will
illuminate.
When the master caution light is pressed, the circuit is
rearmed and the MASTER CAUTION and fuel
annunciate lights are extinguished. Pressing the fuel
annunciate lights provided the low pressure condition
remains (Ref Chapter 33 -Master Warning and Caution
Lights).
When the fuel in the center tank is depleted, the center
tank boost pump low pressure lights are illuminated.
The low pressure lights for the center tank boost pumps
are extinguished when the boost pumps are turned
OFF. The low pressure lights for the No. 1 or 2 tank
boost pumps are illuminated when low fuel pressure
exists or when the boost pumps are turned OFF.

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Figure 43 - Fuel Feed Low Pressure Indicating System

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FUEL TEMPERATURE INDICATING SYSTEM

The temperature indicator displays the fuel

temperature in tank No. 1.

The indicator is located on the P5 panel.

The fuel temperature indicator is resistance ratio
meter type unit. It consists of a no-power sweep-off
coil, deflection coil, cabin temperature compensator,
dial calibrated from -56°C to 56°C and integral lights.
Power
Power supply is 28 Volt AC.
Operation
The indicator rectifies the ac power to dc and applies
it to the metering circuits through an electromagnetic
sweep-off mechanism. This forces the meter to
respond to the coil torques. The indicator applies a
voltage to the temperature bulb and the resulting
current is a measure of the fuel temperature. With
power off, the pointer remains off scale at the low
temperature end.

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Figure 44 - Fuel Temperature Indicating System

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