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Center tank

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Right main tank.

Surge tanks collect fuel overflow. The fuel overflow drains into the main tanks. There are three dry bays (left wing, right wing, center) in the fuel tanks. The dry bays do not hold fuel.
Component Locations
The left main tank is in the wing box of the left wing. The right main tank is in the wing box of the right wing. An adjacent surge tank is outboard of each main tank. The wing dry bays are above each engine strut.
The center tank is in the inboard wing box of the left and right wings. The center dry bay divides the center tank into two parts (left and right). Two interconnect tubes connect the left and right halves of the center tank. The halves operate as one tank when the fuel level is above the interconnect tubes.
Capacity
The capacity of each main tank is approximately 62,870 lb (28 515 kg) of fuel. The capacity of the center tank is approximately 83,825 lb (38 020 kg).
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777 AIRCRAFT MAINTENANCE MANUAL

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General
The fuel tanks are sealed parts of the wing structure. These are the parts of the fuel tanks:
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Upper wing skin

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Lower wing skin

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Front spar

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Rear spar

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Wing ribs.

Wing ribs divide the fuel tanks into bays, and reduce the movement of fuel during airplane maneuvers. Access doors and cutouts are entrances into the airplane fuel tanks for inspection or component repair.
Fuel Tank Locations
The center fuel tank is between rib 8 of the left wing and rib 8 of the right wing. Wing ribs divide each side of the center tank into seven fuel bays. The center wing section between the left and right side-of-body ribs (rib 1) is dry. Three spanwise beams between the left and right side-of-body ribs divide the center dry bay into four bays. A purge door for the center dry bay is on the rear spar.
The main tanks are between rib 8 and rib 32 in each wing. Wing ribs divide each main tank into 24 fuel bays. The forward parts of the bays between rib 8 and rib 11 are dry. These wing dry bays prevent a fuel spill on the engine if an engine burst occurs.
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The surge tanks are between rib 32 and rib 34 in each wing. A wing rib divides each surge tank into two fuel bays.
Wing Ribs
Standard wing ribs have upper and lower openings between the stringers that let fuel and air flow through them.
Tank-end ribs close the ends of each fuel tank. Tank-end ribs are sealed. No fuel can flow through them.
Rib 18 is a baffle rib. The rib is sealed, but has check valves between each lower stringer. The check valves let fuel flow inboard at a slow rate. One hole at the bottom of the rib lets fuel flow outboard. This keeps the fuel level on the two sides of the rib the same. Two large holes and six smaller holes at the top of the rib let air through it.
Rib 10 is a fuel dam rib. The lower section is the same as the baffle rib, but the upper section is the same as a standard rib. The fuel dams of rib 10 prevent fuel from flowing away from the fuel pump inlets.
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Fuel Tank Access Doors
The access doors fit in fuel bays over openings in the airplane skin. A clamp ring and knitted aluminum gasket bolt to the access door from outside the airplane to hold the access door in its position. Thus, no bolt holes are in the skin. The gasket makes an electrical bond between the access door and the airplane skin. A rubber seal in the access door prevents fuel leaks around the access door.
Six of the access doors in each main fuel tank have fuel quantity measuring stick assemblies. The eleven inboard access doors of each wing have impact-resistant access doors.
Rib and Spanwise Beam Cutouts
Some of the bays do not have an access door. You can get access to these bays from adjacent bays. Access openings in wing ribs and spanwise beams let you go in from an adjacent bay.
The purge door is in the left main wheel well. It supplies an opening for airflow when you purge the center dry bay.
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