Fueling VALVE POSITION LIGHTS
Extinguished –
.
fueling valve switch is OPEN and related tank is full

.
fueling valve switch is CLOSED. Illuminated (blue) – fueling valve switch is OPEN and related tank is not full.

Fueling Valve Switches
OPEN – energizes fueling valve in related tank.
CLOSED – de–energizes fueling valve in related tank.

FUEL Quantity Indicators
Indicates total usable fuel tank quantity in related tank.

AUX TANK Fueling Valve Switch
Inoperative.
Fuel -Controls and Indicators 737 Operations Manual

Intentionally
Blank

Copyright . The Boeing Company. See title page for details.
12.10.10  December 1, 2000

737 Operations Manual

Introduction
The fuel system supplies fuel to the engines and the APU. Fuel is contained in

three tanks located within the wings and wing center section.
Refer to Engine and APU chapter for a description of the engine and APU fuel
systems.

Fuel Feed
Both engines are normally pressure fed from the center tank until the center tank quantity decreases to near zero. The engines are normally then pressure fed from their respective main tanks. Check valves are located throughout the fuel system to ensure the proper direction of fuel flow and to prevent transfer of fuel between tanks.
Fuel Pumps
Each fuel tank uses two AC powered fuel pumps which are fuel cooled and lubricated. Center tank check valves open at a lower pressure than do the main tank check valves. This ensures that center tank fuel is used before main tank fuel, even though all fuel pumps are operating. Individual pressure sensors monitor the output pressure of each pump.
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.
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.
The main tank bypass valves may also be used for suction defueling.
Fuel Crossfeed
The engine fuel manifolds are interconnected by use of the crossfeed valve. The valve is DC motor operated from the battery bus.
Fuel -System Description

737 Operations Manual
Fuel pressure can be provided from a main tank with operating fuel pumps to both engines by opening the fuel crossfeed valve. Continued crossfeed use will result in a progressive fuel imbalance.
Fuel Shutoff Valves
Fuel shutoff valves are located at the engine–mounting wing stations. The valves are DC motor operated from the hot battery bus. They close whenever the respective engine fire warning switch is pulled or engine start lever is placed to CUTOFF.
Center Tank Scavenge Jet Pump
When both center tank fuel pump switches are turned OFF, the fuel scavenge shutoff valve opens. This allows fuel pressure 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.
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 pressures 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.
Fuel Temperature
The FUEL TEMP indicator located on the fuel control panel displays fuel temperature. A sensor in main tank No. 1 allows monitoring of fuel temperature. The temperature indicating system uses AC electrical power.
APU Fuel Feed
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.
DC Operated APU Fuel Pump
On some airplanes a DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. The pump operates automatically during APU start and shuts off automatically once the APU is at governed speed, ready to accept a load.
Copyright . The Boeing Company. See title page for details.
12.20.2  December 6, 2002

Fuel -System Description 737 Operations Manual
Fueling/Defueling/Ground Transfer
Rapid fueling and defueling is accomplished at the single–point pressure fueling station in the right wing. The fueling station is also used for the ground transfer of fuel between tanks.
　
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