H.  
Maintenance for the scavenge system is done by the removal of the two fuel strainers attached to the rear spar in the wheel well for the wing landing gear.

 (1)  
With the fuel strainers removed, a poppet valve in the fuel strainer housing prevents fuel flow into the fuel scavenge system.

 I.  
The hydromechanical system has these components:

 (1)  
Four jet pumps in the center wing tank

 (2)  
Two float operated shutoff valves in the center wing tank

 (3)  
One float operated shutoff valve in the No.2 main tank

 (4)  
One float operated shutoff valve in the No.3 main tank

 (5)  
Two fuel strainer housing assemblies on the rear spar of the center wing tank

 (6)  
Two fuel strainers which are contained in the fuel strainer housing

 (7)  
Fuel tubing, couplings, two fuel inlet screens and related components that attach the fuel scavenge system to the structure.

 2.  Hydromechanical Fuel Scavenge System (Fig. 1)____________________________________
 A.  Fuel Scavenge Jet Pumps
 (1)  
There are four jet pumps in the center wing tank. The jet pumps are attached to the lower panel intercostals (forward and aft of the mid spar of the center wing tank). To get to the jet pumps you must go through the center wing tank access door and the spanwise beam access doors.

 (2)  
The jet pumps are used to move the fuel from the center wing tank to the inboard main tanks. The jet pumps use pressurized fuel flow from the fuel-feed transfer line to supply the motive port with fuel pressure. The motive port pulls a secondary fuel flow from the fuel inlet on the bottom of the center wing tank through the jet pump induced port. The motive and induced flow are mixed together and then exit the jet pump through the discharge port. The fuel flows through a tube attached to the wing center section mid spar into the No. 2 or the No. 3 main tank. A check ball in the jet pump closes when the motive flow pressure is low to stop the flow of fuel back into the center wing tank.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

 B. Float Valve in the Center Wing Tank
 (1)  
There are two float valves in the center wing tank attached to the lower intercostals (aft of the No. 1 spanwise beam for the center wing tank). The float valves are found near the bottom of the tank. To get to the float valves you must go in the center wing tank through the center wing tank access door and the spanwise beam access doors.

 (2)  
The valve contains a body and a float assembly. The body contains a poppet and a stem which is attached to the float.

 (3)  
When the center wing tank is empty, the float is down which causes the stem to pull the poppet and the valve open. When the level of the fuel in the center wing tank is above the float valve, the float moves up which causes the poppet and the valve to close. When the float valve is closed, fuel cannot move from the fuel-feed transfer line into the fuel scavenge system.

 C.
Float Valve in the No. 2 and No. 3 Main Tanks

 (1)  
There is one float valve in each of the No. 2 and No. 3 inboard main tanks attached to the first rib ((WBL 150.00) at the mid spar. The float valve is found near the top of the fuel tank. To get to the float valves in the No. 2 or No. 3 main tanks, you must go into the inboard main tank through access doors 544AB or 544AB.

 (2)  
The valve contains a body and float assembly. The body contains a poppet and a stem which is attached to the float.

 (3)  
When the inboard main tanks are not full, the float is down which causes the stem to pull the poppet and the valve open. When the level of the fuel in the inboard main tank is above the float valve, the float moves up which causes the poppet and the valve to close. When the float valve is closed, fuel cannot go through the scavenge system from the center wing tank to the inboard main tanks. This prevents an overfill condition in the inboard main tanks. The closed float valve in the main tanks also prevents the flow of fuel from the main tanks back into the center wing tank through the fuel scavenge system line.

 D.
Fuel Strainer Housing

 (1)  
　
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