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BOEING 737 SYSTEMS REVIEW
Page 1
FUEL
1. GENERAL
¡ Fuel is contained in three tanks located within the wings and wing center section.
¡ There a 3 tanks : Main tanks No. 1 and No. 2 are integral with the wing structure. The center tank
lies between the wing roots within the fuselage area and extends out into the wing structure.
¡ Each tank is equipped with two electrical fuel boost pumps which supply fuel directly to the
respective engine or to both engines through the fuel crossfeed valve.
¡ Mechanical engine-driven fuel pumps also provide suction fuel feed from the two main tanks. Fuel
for APU operation is normally supplied from the left side of the fuel manifold.
2. FUEL FEED
¡ Two AC powered fuel pumps, which are fuel cooled and lubricated, are installed for each fuel tank.
B737 Classics : A single generator failure will only affect one pump in each tank.
¡ Individual pressure sensors monitor the output pressure of each pump.
¡ The engine-driven fuel pumps provide suction feed in the event that normal electrical fuel pump
operation is not available. The engine pumps draw fuel through bypass valves located in main tanks
No. 1 and No. 2. No bypass valve is available in the center tank. The main tank bypass valves may
also be used for suction defueling.
¡ The engine fuel shutoff valves (at each engine-mounting wing station) and 737 NG spar valve are DC
motor operated from the hot battery bus. They close whenever the respective Engine Fire Warning
Switch is pulled or the Engine Start Lever is placed to CUTOFF.
¡ The engine fuel manifolds are interconnected by the crossfeed valve directing fuel to both engines
from any tank. The valve is DC motor operated from the battery bus.
¡ Check valves ensure the proper direction of fuel flow and prevent transfer of fuel between tanks.
- 737 Classics : Center tank check valves open at a lower differential pressure than the check valves
in main tanks No. 1 and No. 2.
- 737 NG : Center tank pumps produce higher pressure than main tank pumps.
This ensures that center tank fuel is used before main tank fuel, even though all fuel pumps are
operating.
¡ When AC fuel pumps are operating, fuel for the APU is supplied from the left side of the fuel manifold.
If the AC fuel pumps are not operating, fuel is suction fed from main tank No. 1.
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BOEING 737 SYSTEMS REVIEW
3. CENTER TANK SCAVENGE JET PUMP
737 Classics
¡ When both Center Tank Fuel Pump Switches are turned OFF, the fuel scavenge shutoff valve opens.
This allows fuel from the main tank No. 1 forward pump to operate the Center Tank Scavenge Jet
Pump, which transfers the remaining center tank fuel to main tank No. 1. After 20 minutes, the fuel
scavenge shutoff valve automatically closes.
¡ When any Center Tank Fuel Pump Switch is subsequently recycled ON then OFF, the Center Tank
Scavenge Jet Pump will again be operated for 20 minutes.
737-800
¡ The center tank fuel scavenge jet pump begins to operate when the No. 1 main tank is about 2/3 full
and the main tank No. 1 forward pump is operating. Once the fuel scavenging process starts, it
continues for the remainder of the flight.
4. FUEL VENT SYSTEM
¡ The purpose of the fuel vent system is to prevent damage to the wings due to excessive buildup of
positive or negative pressure inside the fuel tanks and to provide ram air pressure within the tanks.
The tanks are vented into surge tanks which vent through a single opening at each wing tip.
5. FUEL TEMPERATURE
¡ A sensor in main tank No. 1 allows monitoring of fuel system temperature.
indicating system uses AC electrical power.
The temperature
6. TANK CAPACITY USEABLE FUEL
737 Classics
TANK
No. 1
No. 2
Center tank
TOTAL
LITERS
5667
5667
8743
20077
WEIGHT (kg)
4530
4530
7000
16060
TANK
No. 1
No. 2
Center tank
TOTAL
LITERS
4876
4876
16273
26025
WEIGHT (kg)
3900
3900
13020
20820
737 NG
The above figures are approximate amounts of fuel. The appropriate weight and balance control &
loading manual gives exact figures for all configurations. Fuel density used - 0.80 kg / liter.
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BOEING 737 SYSTEMS REVIEW
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7. FUELING / DEFUELING / GROUND TRANSFER
¡ Fueling and defueling is accomplished at the fueling station in the right wing. The fueling station is
also used for the ground transfer of fuel between tanks.
¡ A shutoff system automatically closes the fueling valve in each fuel tank when the tank is full.
¡ 737 Classics : Overwing fueling receptacles for main tanks No. 1 and No. 2 are provided for gravity
fueling. Center tank servicing can only be accomplished through the ground tank-to-tank fuel
transferring operation.
¡ The manual defueling valve interconnects the engine feed system and the fueling station.
It is opened for defueling and tank-to-tank transfer operations.
8. CONTROLS & INDICATORS
Fuel Control Panel IN
DICATOR
¡ TEMPERATURE INDICATOR : Indicates fuel temperature in main tank No. 1.
¡ CROSSFEED SELECTOR : controls the fuel crossfeed valve.
When in OPEN position : connects the engine No. 1 and No. 2 fuel feed lines.
¡ FUEL PUMP SWITCH ON : activates the fuel pump.
Fuel Quantity Indicators
¡ FUEL QUANTITY INDICATOR
ILLUMINATED (white):
- Indicates useable fuel in the related tank.
- Accuracy is 2.5% of full scale reading
- Standby AC power is required.
¡ FUEL QUANTITY TEST SWITCH (B737 Classics only) : used for maintenance purposes only.
¡ ERROR INDICATOR
ERR :
- Appears with associated error codes whenever a malfunction occurs.
- Used normally for maintenance purposes only.
ERR
CODE
1
2
3
4
5
6
7
8
9
10
FQI Reading
Probable cause
Normal
zero
Normal
Zero
Zero
Normal
*(1)
Blank
*(1)
Zero
Open or short
Short in compensator
Excessive compensator unit leakage
Open circuit
Short in tank unit
Excessive tank unit leakage
Calibration unit failure
Error in DCTU data
Indicator Memory failure
Open or short
*(1) - if failure occurs during flight indicator reading will be normal with ERR shown
- if failure occurs when power is applied, the FQI will read zero with ERR shown
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BOEING 737 SYSTEMS REVIEW
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¡ LOW FUEL INDICATION (B737 NG only)
ILLUMINATED (amber):
- Fuel quantity is less than 907 kg (2000 lbs) in related main tank.
- Fuel quantity arc and digits on tank(s) with low fuel quantity turn amber.
- Displayed until quantity is increased to 1134 kg (2500 lbs).
¡ FUEL CONFIGURATION (CONFIG) INDICATION (B737 NG only)
ILLUMINATED (amber):
- Center tank quantity greater than 726 kg (1600 lbs); both center tank pumps are producing low or
no pressure and either engine is running.
- Fuel quantity arc and digits on center tank turn amber.
- Inhibited when center tank quantity is less than 363 kg (800 lbs).
¡ FUEL IMBALANCE (IMBAL) INDICATION (B737 NG only)
ILLUMINATED (amber):
- Main tanks differ more than 453 kg (1000 lbs).
- Fuel quantity arc and digits on tank with lower fuel quantity turn amber.
- Inhibited on ground.
- Inhibited by LOW fuel indication when both indications exist.
- Displayed until imbalance is reduced to 91 kg (200 lbs) .
External Fueling Panel
¡ SOLENOID OVERRIDE (red)
- Mechanically opens the solenoid operated valve.
- The fuel valve opens if fuel pressure is available.
¡ FUELING VALVE : With the Battery Switch ON, fuel pressure opens the valve, if energized.
¡ FUELING POWER CONTROL SWITCH
DOOR CLOSED :The proximity sensor deactivates power of the fueling system.
DOOR OPEN :
- The fueling system is powered.
- The panel lights illuminate.
¡ FUELING VALVE POSITION LIGHT (blue)
EXTINGUISHED :
- The fueling Valve Switch is in the open position and the associated tank is full.
- The Fueling Valve Switch is in the closed position.
ILLUMINATED :
- The Fueling Valve Switch is in the open position and the associated tank is not full.
¡ FUELING VALVE SWITCH
OPEN :
- Energizes the fueling valve.
CLOSED :
- De-energizes the fueling valve.
¡ FUEL QUANTITY INDICATOR : indicates total useable fuel tank quantity.
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BOEING 737 SYSTEMS REVIEW
Page 5
Fuel Measuring Stick
¡ Either dripsticks or floatsticks are installed.
- Allows a comparison of fuel quantity or weight determined from the stick reading with the fuel
weight indicated by the Fuel Quantity Indicators.
- Five fuel measure sticks are installed in each main tank.
¡ DRIPSTICK:
A dripstick reading is obtained by withdrawing the dripstick from the tank until a steady drip of fuel
commences at the drip hole near the base.
¡ FLOATSTICK :
- A floatstick reading is obtained by withdrawing the flexible floatstick scale from the tank until the
scale ,"sticks" or "hangs".
- The floatstick indication is read from the floatstick scale level with the lower wing skin and corrected
for aircraft attitude via conversion/correction tables.
Center Panel
¡ FUEL FLOW SWITCH
RATE : the digital fuel flow are displayed on both fuel flow indicators. They show rate of fuel
Consumption (kg/hr or lbs/hr)
USED : - displays fuel used per engine since last reset (B737 Classics)
- displays fuel used per engine since last reset after l0 seconds , display automatically
reverts to fuel flow if switch is activated longer than 30 sec, fuel used readout is
deactivated for remainder of flight (B737 NG)
RESET : - decreases readout to zero (B737 Classics)
- resets fuel used to zero (B737 NG) displays fuel used for 1 second, decreases to zero,
then displays fuel flow
Note: failure of any N1, EGT, N2 or Fuel Flow indicator or transmitter results in the respective digital
display blanking and the pointer moving to zero
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BOEING 737 SYSTEMS REVIEW
9. FAULTS & INDICATIONS
VALID FOR
LIGHT
ENG VALVE
CLOSED
FUEL VALVE
CLOSED
FILTER
BYPASS
LOW
PRESSURE
INDICATION
ENGINE VALVE CLOSED LIGHT (blue)
EXTINGUISHED :
- The engine fuel shutoff valve is open.
ILLUMINATED (bright);
- The engine fuel shutoff valve is in transit, or the
position of the valve is not in agreement with the
position of the associated Start Lever or Engine Fire
Warning Switch.
ILLUMINATED (dim):
- The engine fuel shutoff valve is closed.
FUEL VALVE CLOSED LIGHT (blue)
EXTINGUISHED :
- The engine fuel shutoff valve is open.
ILLUMINATED (bright);
- The engine fuel shutoff valve is in transit, or the
position of the valve is not in agreement with the
position of the associated Start Lever or Engine Fire
Warning Switch.
ILLUMINATED (dim):
- The engine fuel shutoff valve is closed.
FILTER BYPASS LIGHT (amber)
- Indicates impending or actual filter bypass due to a
contaminated filter.
FUEL PUMP LOW PRESSURE LIGHT* (amber)
* MAIN TANK FUEL
ILLUMINATED :
Fuel pump output pressure is low, or the Fuel Pump
Switch is OFF.
300
X
400
X
500
600
700
800
900
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
NOTE : Two LOW PRESSURE Lights illuminated in the same tank
will illuminate the MASTER CAUTION and FUEL System
Annunciator Lights. One LOW PRESSURE Light will cause the
MASTER CAUTION and FUEL System Annunciator Lights to
illuminate on MASTER CAUTION Light recall.
LOW
PRESSURE
EXTINGUISHED :
Fuel pump output pressure is normal.
FUEL PUMP LOW PRESSURE LIGHT* (amber)
* CENTER TANK FUEL
ILLUMINATED:
Fuel pump output pressure is low and the Fuel Pump
Switch is ON.
NOTE :
- With both Center Tank Fuel Pump Switches ON, the illumination
of both LOW PRESSURE Lights will illuminate the Master Caution
and the FUEL System Annunciator Lights. The illumination of one
LOW PRESSURE Light will illuminate the MASTER CAUTION and
FUEL System Annunciator Lights on MASTER CAUTION Light
recall.
- With one Center Tank Fuel Pump Switch
OFF, the illumination
of the opposite Center Tank LOW PRESSURE Light will illuminate
the MASTER CAUTION and Fuel System Annunciator Lights.
EXTINGUISHED :
Fuel pump output pressure is normal.
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SPAR VALVE
CLOSED
Page 7
BOEING 737 SYSTEMS REVIEW
SPAR VALVE CLOSED LIGHT (blue)
EXTINGUISHED:
- The spar valve is closed.
X
X
X
X
X
X
ILLUMINATED (bright):
- The spar valve is in transit, or the position of the
valve is not in agreement with the position of the
associated Start Lever or Engine Fire Warning
Switch.
ILLUMINATED (dim):
- The spar valve is open.
VALVE
CLOSED
CROSSFEED VALVE CLOSED LIGHT (blue)
EXTINGUISHED :
- The crossfeed valve is closed.
ILLUMINATED (bright):
- The crossfeed valve is in transit, or the position of
the valve is not in agreement with the position of the
Crossfeed Selector.
ILLUMINATED (dim):
The crossfeed valve is open.
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X
X
X
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BOEING 737 SYSTEMS REVIEW
FUEL SCHEMATIC B737 CLASSICS
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BOEING 737 SYSTEMS REVIEW
FUEL SCHEMATICS B737 NG
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