S 644-008

 (2)  
Lightly lubricate the new O-ring with petrolatum.

 S 434-014

 (3)  
Put the O-ring into the vent tube couplings.

 S 434-009

 (4)  
Hold the fuel vent tube into its position.

 S 424-013

 (5)
Tighten the couplings and install a lockwire into the couplings.

 S 434-010

 (6)  
Install a bonding clamp on the vent tube and connect the bonding jumper (Ref 20-11-15/401).

 (7)
Install the access door 556AB or 656AB for the wingtip dry bay.

 S 414-011
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

 28-13-09
 ALL  ú ú 01 Page 403 ú Jun 10/91
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 CENTER WING TANK SCAVENGE SYSTEM - DESCRIPTION AND OPERATION
____________________________________________________________
 1.  General_______
 A.  The hydromechanical fuel scavenge system is a non-electrical system that uses four jet pumps to move (scavenge) the fuel that cannot be pumped by the CWT override/jettison pumps. The fuel is scavenged from the center wing tank into the No. 2 and No. 3 main tanks.
 B.  There is a left side scavenge system that goes into the No. 2 main tank and a right side system that goes into the No. 3 main tank. The operation of the left and right side of the scavenge system is the same.
 C.  With all four of the jet pumps below the level of the fuel, the scavenge system moves fuel from the center wing tank to the inboard main tanks at an approximate rate of 1200 lbs (545 kgs) per hour.
 D.  The scavenge system operates when:
 (1)  
The fuel-feed transfer line is pressurized by one or more of the boost pumps or the override/jettison pumps and:

 (a)  
the float valve(s) near the bottom of the center wing tank are open (the approximate CWT fuel quantity is less than 4000 lbs/ 1820 kgs)

 (b)  
the float valve(s) on the top of the inboard main tanks are open (the approximate M2 or M3 fuel quantity is less than 60,000 lbs/27,270 kgs).

 E.  
When the float valves in the inboard main tanks are 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.

 F.  
On the ground, the fuel-feed transfer line is pressurized during APU or fuel transfer operations. If the scavenge system floats in the center wing tank and the inboard main tank are open, fuel will move from the center wing tank to the inboard main tanks.

 (1)  
During APU operation the aft boost pumps in the No. 2 and No. 3 main tanks supply pressure to the APU fuel supply line through the fuel-feed transfer line. The automatic operation of the aft boost pumps in the No. 2 and No. 3 main tanks prevent a fuel imbalance if the fuel scavenge system operates.

 (2)  
During APU operation if the No. 1 or 4 main tanks or the center wing tank is used to supply fuel to the APU, a fuel imbalance will not occur, but fuel will move (transfer) from the center wing tank to the No. 2 and No. 3 main tanks if the scavenge system operates.

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 28-15-00
 ALL  ú ú 06B Page 1 ú Feb 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 (3)  
When fuel is moved from one fuel tank to another tank (fuel transfer) the boost pumps and/or the override/jettison pumps supply pressure to the the fuel-feed transfer line. If the fuel scavenge system operates, unwanted fuel can move into the No. 2 and No. 3 main tanks. To prevent the transfer of fuel into the No. 2 or No. 3 main tanks, it is necessary to remove the left side fuel strainer for the No. 2 main tank and the right side fuel strainer for the No. 3 main tank (AMM 28-15-00/201).

 G.  
The fuel synoptic can be used to check the operation of the scavenge system. If there is a decrease in the fuel quantity in the center wing tank with a subsequent increase in the No. 2 or No. 3 main tanks, the scavenge system is operational.
　
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