B757 Fuel Boeing B757 - Systems Summary [Fuel]
12.20 Fuel-System Description
Introduction
The fuel system supplies fuel to the engines and the APU. The fuel is contained in
a center tank, and left and right main tanks.
Fuel Quantity
Fuel quantity data, measured by probes in each tank, is fed to the fuel quantity
processor where it is corrected for density then displayed on a fuel quantity
indicator for each tank. Total fuel quantity, from a separate calculation, is shown
on the total fuel quantity indicator and is also provided to the FMC.
When total usable fuel in either the left or right main tank drops below
approximately 2200 pounds, the FUEL CONFIG light illuminates and the LOW
FUEL caution message is displayed.
Fuel Temperature
Temperature of the fuel in the right main tank is displayed on the fuel temperature
indicator.
Fuel Pumps
Each fuel tank contains two AC–powered fuel pumps. A single pump can supply
sufficient fuel to operate one engine under all conditions.
The two center tank fuel pumps have greater output pressure than the left and right
main tank fuel pumps. When all six pumps are operating, the center tank pumps
override the left and right main tank pumps so that center tank fuel is used before
left or right main tank fuel.
If any pump has low output pressure, the appropriate switch PRESS light
illuminates and the pump pressure EICAS message is displayed. If the left or right
main tank pump switches are OFF, the low pressure lights are illuminated and
EICAS messages for the pumps are displayed. When the center pump switches are
OFF, the low pressure lights and EICAS messages for the center pumps are
inhibited.
To reduce electrical loads, the center tank pumps are inhibited when the
associated N3 is less than 50% RPM. Thus both center tank pumps are inhibited
when the engines are shutdown. As an engine is started and N3 RPM increases
above 50%, the inhibit is removed for the associated center tank pump.
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Boeing B757 - Systems Summary [Fuel]
To reduce electrical loads, the center tank pumps are inhibited when the associated
N2 is less than 50% RPM. Thus both center tank pumps are inhibited when the
engines are shutdown. As an engine is started and N2 RPM increases above 50%,
the inhibit is removed for the associated center tank pump.
The EICAS caution messages, L or R FUEL SYS PRESS, displays when all fuel
pumps have low output pressure or all fuel pumps on one side have low output
pressure and the crossfeed switches are off.
The fuel pump low pressure messages are inhibited by the corresponding L or R
FUEL SYS PRESS messages.
During normal operation, the EICAS advisory messages CTR L FUEL PUMP and
CTR R FUEL PUMP display to indicate depletion of center tank fuel.
With either messages displayed, a small amount of center tank fuel maybe
indicated. A scavenge system will operate automatically to transfer any remaining
center tank fuel to the left main tank. Fuel scavenge begins when the left main tank
is approximately half empty.
The left main tank contains a DC–powered fuel pump. It has no controls or
indicators. The DC pump operates automatically to provide fuel to the APU when
AC power is not available and the APU selector is ON.
Fuel Crossfeed
The fuel manifolds are arranged so that any fuel tank pump can supply either
engine. Two crossfeed valves isolate the left fuel manifold from the right. These
valves are normally closed providing fuel feed from tank to engine. Both valves
are opened any time it becomes necessary to feed an engine from an opposite fuel
tank. Only one open crossfeed valve is required for successful crossfeed
operation. A VALVE disagreement light illuminates and the EICAS advisory
message FWD FUEL X–FEED or AFT FUEL X–FEED displays if a valve
position does not agree with its switch position. The L or R FUEL SYS PRESS
messages are inhibited with either crossfeed valve open.
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Boeing B757 - Systems Summary [Fuel]
Suction Feed
When main tank fuel pump pressure is low, each engine can draw fuel from its
corresponding main tank through a suction feed line that bypasses the pumps. As
the airplane climbs, dissolved air is released from the fuel in the tank due to the
decrease in air pressure. This air may collect in the suction feed line and restrict
fuel flow. At high altitude, thrust deterioration or engine flameout may occur as a
result of the fuel flow reduction.
Fuel pressure can be provided from a main tank with operating fuel pumps to both
engines by opening the fuel crossfeed valves. Continued crossfeed use will result
in a progressive fuel imbalance.
The dissolved air in the fuel tank will eventually deplete after reaching cruise
altitude. The depletion time is dependent upon airplane altitude, fuel temperature,
and type of fuel. Once the dissolved air is depleted, the engine may be capable of
suction feed operation at cruise power.
Fuel Configuration Light
When the fuel quantity in left and right main tanks differ by 1800 pounds or center
fuel pump switches are OFF with more than 1200 pounds in the center tank, the
FUEL CONFIG light illuminates and the EICAS advisory message FUEL
CONFIG is displayed.
The FUEL CONFIG light also illuminates when the LOW FUEL EICAS message
is displayed.
Fuel Imbalance
Fuel balancing is accomplished by opening the crossfeed valves and turning off
the fuel pump switches for the left or right main fuel tank that has the lowest
quantity. Fuel balancing may be done in any phase of flight.
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Boeing B757 - Systems Summary [Fuel]
Fuel Tank Locations and Capacities
Fuel Tank Locations
RIGHT MAIN TANK
CENTER TANK
LEFT MAIN TANK
Fuel Tank Capacities
Total
Tank
U.S. Gallons
Pounds *
Left main
2,176
14,579
Right main
2,176
14,579
Center
6,924
46,391
11,276
75,549
* Usable fuel at level attitude, fuel density = 6.7 LB/U.S. Gallon
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Boeing B757 - Systems Summary [Fuel]
Fuel System Schematic
L
PUMPS
AFT
FWD
FUEL
XFEED
ON
PRESS
VALVE
ON
PRESS
VALVE
C PUMPS
L
ON
P
R
E
S
S
ENGINE
VALVE
SPAR
VALVE
CENTER
CROSSFEED
VALVES
LEFT
MAIN
DC
APU FUEL
VALVE
APU
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Boeing B757 - Systems Summary [Fuel]
APU Fuel Feed
APU fuel is supplied from the left fuel manifold. APU fuel can be provided by any
AC fuel pump supplying fuel to the left fuel manifold or by the left main tank DC
fuel pump.
On the ground, with the APU selector ON and no AC power available, the DC
pump runs automatically. With AC power available, the left forward AC fuel
pump operates automatically, regardless of fuel pump switch position, and the DC
fuel pump turns off. In flight, the left forward AC fuel pump can be turned off with
the fuel pump switch.
Fuel System FMS CDU Messages
The CDU can display the following messages:
INSUFFICIENT FUEL – Predicted fuel at destination is less than the FMC
reserves
FUEL DISAGREE–PROG 2 – The fuel totalizer and calculated fuel quantity
disagree
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Boeing B757 - Systems Summary [Fuel]
12.30 Fuel-EICAS Messages
Fuel System EICAS Messages
The following EICAS messages can be displayed.
Message
Level
Light
Aural
FUEL CONFIG
Advisory
FUEL
CONFIG
Both center pump
switches are OFF with
fuel in the center tank, or a
fuel imbalance between
left and right main tanks
AFT FUEL X-FEED
Advisory
VALVE
The crossfeed valve
position disagrees with the
commanded position
Advisory
PRESS
Fuel pump output pressure
is low
FWD FUEL X-FEED
CTR L FUEL PUMP
CTR R FUEL PUMP
Condition
L AFT FUEL PUMP
R AFT FUEL PUMP
L FWD FUEL PUMP
R FWD FUEL PUMP
L FUEL SYS PRESS
Caution
Beeper
All fuel pumps have low
output pressure or all fuel
pumps on one side have
low output pressure and
the crossfeed switches are
off
Beeper
Fuel quantity is low in
either left or right main
tank
R FUEL SYS PRESS
LOW FUEL
Caution
FUEL
CONFIG
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Boeing B757 - Systems Summary [Fuel]
12.10 Fuel-Controls and Indicators
Fuel System
L
PUMPS
1
2
AFT
FWD
FUEL
XFEED
R
PUMPS
VALVE
PRESS
VALVE
PRESS
ON
ON
PRESS
ON
ON
PRESS
L
ON
P
R
E
S
S
AFT
FWD
5
C PUMPS
P
R
E
S
S
4
FUEL
CONFIG
ON
R
3
OVERHEAD PANEL
1
Left/Center/Right (L/C/R PUMPS) Switches
ON – the fuel pump is selected ON
Off (ON not visible) – the fuel pump is selected off
2
Left/Right Pump Pressure (PRESS) Lights
Illuminated (amber) – fuel pump output pressure is low
3
Center Pump Pressure (PRESS) Lights
Illuminated (amber) –
• fuel pump output pressure is low with the pump selected ON
• associated N3 below 50% with pump switch ON
• associated N2 below 50% with pump switch ON
4
Fuel Crossfeed (FUEL XFEED) Switches
On (bar visible) – the crossfeed valve is selected open
Off (bar not visible) – the crossfeed valve is selected closed
5
Crossfeed VALVE Light
Illuminated (amber) – the crossfeed valve is not in the selected position
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6
Boeing B757 - Systems Summary [Fuel]
Fuel Configuration (FUEL CONFIG) Light
6
Illuminated (amber) –
• low fuel quantity
• imbalance between left and right main tanks
• center tank fuel pumps off with fuel in center tanks
Fuel Indications
Fuel Quantity Indicator
C
L
R
1
LB
LB
LB
FUEL QTY
2
3
X 1000
TEMP
oC
TOTAL
OVERHEAD PANEL
1
Fuel Quantity (L/C/R FUEL QTY) Indication
Displays usable fuel quantity in the left main, center, and right main tank (pounds
x 1000)
2
Fuel Temperature (TEMP) Indication
Displays temperature of fuel in the right main tank (degrees celsius)
3
TOTAL Fuel Quantity Indication
Displays total usable fuel quantity in all tanks (pounds x 1000)
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Boeing B757 - Systems Summary [Fuel]
Fuel Quantity Test
1
FUEL
QTY
ACCESSORY PANEL
1
Fuel Quantity (FUEL QTY) Test Switch –
Spring–loaded to center
Initiates fuel quantity test
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