FUEL STORAGE -GENERAL DESCRIPTION -TANK STRUCTURE AND ACCESS -2

General
You remove access doors on the bottom of the tanks and dry bays to get access inside the tanks.
Center Wing Section Access
You get access into the center dry bay through a single access door. The access door (139AZ) is on the forward, center, lower skin of the center dry bay. You must open panel 139AL on the wing-body fairing to get to center dry bay access door 139AZ.
Spanwise beams in the center wing section have access openings. You get access to each bay through the spanwise beam openings.
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FUEL STORAGE -GENERAL DESCRIPTION -TANK STRUCTURE AND ACCESS -2

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FUEL STORAGE -SUMP DRAIN VALVES

General
Sump drain valves are at the low point of each fuel tank.
You use the sump drain valves to these functions:
*
Get fuel samples

*
Remove water and contaminants from the fuel tanks

*
Drain all the fuel that remains after the defuel procedure

*
Check for fuel in a surge tank before you open an access door.

Component Locations
The center tank has two sump drain valves. There is a valve in the lower wing skin near each wing-to-body fairing. The low points of the center tank are at the wing root on the left and right sides.
Each main tank has one sump drain valve. The sump drain valve is in the lower wing skin near the fourth access door from the fuselage (between rib 8 and rib 9).
A sump drain valve for each surge tank is between rib 32 and rib 33 (26 access doors away from the fuselage).
Functional Description
Each sump drain valve has a primary poppet and a secondary poppet (not shown). The secondary poppet lets you remove the primary poppet without defueling.
The low point of the center tank is below the sump drain valve. Thus, the center tank has a drain line that goes from the low point of the tank to the valve. The weight of the fuel in the tank moves the fuel sample through the drain line to the open valve.
Operation
To drain fluid from the center tank, push up on the primary poppet in the center tank sump drain valve with a fuel sampling tool. You must hold the poppet in the open position.
To drain fluid from a main tank or to check for fuel in a surge tank, turn and push up on the primary poppet with a fuel sampling tool. You must hold the poppet in the open position.
To drain the fuel that remains after you do the defuel procedure, you can remove the primary poppet plug and primary poppet. Then install the sump drain fitting/tool to hold the secondary poppet open. This lets all the fuel drain through the sump drain/ fitting tool.
Training Information Point
You should get a fuel sample from the sump drain valves before and after you do the refuel procedure. A large quantity of water in a fuel sample that you drain from a tank before refueling is an indication of a blocked water scavenge jet pump.
Cold weather can freeze the water in the fuel tanks. This can prevent the drain valves from opening.
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777 AIRCRAFT MAINTENANCE MANUAL

FUEL STORAGE -SUMP DRAIN VALVES

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777 AIRCRAFT MAINTENANCE MANUAL

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FUEL STORAGE -TANK VENT SYSTEM

General
The fuel vent system keeps the pressure of the fuel tanks near the pressure of the outside atmosphere. A large pressure difference can damage the wing structure.
Drains let fuel in the vent system return to the fuel tanks so the engines or APU can use it.
Flame arrestors make sure that a flame does not come inside the fuel tanks through the vent system. If a flame arrestor becomes blocked, pressure relief valves in the surge tank open to make a vent.
Component Locations
Hat-section upper stringers and the upper wing skin make vent channels. A vent tube that has a drain valve at the low point attaches to each vent channel.
Each main tank has a forward and an aft vent channel. A fuel vent float valve attaches to the outboard end of each forward vent channel.
The center tank has three vent channels. Two float actuated drain valves attach to the low points on each vent channel.
There is a vent scoop access door in each surge tank. Each vent scoop contains a flame arrestor.
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