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B.  The integral center tank within the wing center section is vented through an opening in the forward cavity vent tube. The vent tube connects with a spanwise duct running to the vent surge tanks. The center tank has a local open-ended loop vent tube at each side of the center tank to vent air trapped on the rear side of the spanwise duct.
C.  During flight, impact pressure is applied to the ram air vent scoop. The pressure is then transmitted through the vent ducts to the expansion space in each tank to maintain a positive pressure on the fuel. Since the expansion space will shift with varying airplane attitudes, the wing tank vent ducts are provided with inboard and outboard port openings to ensure that at least one port will be open to the expansion space in any normal airplane attitude. The inboard port is open at all times, and the outboard port is protected by a float valve, to prevent fuel from entering the vent system during certain airplane attitudes and flowing overboard. Fuel entering the vent system during rapid attitude changes will flow into the vent surge tank. When normal flying attitude is established, the fuel will flow through a surge tank drain line into the center tank vent channel and through a check valve into the center tank. 
D.  If a fueling shutoff valve fails to close during pressure fueling, the vent lines will carry the excess fuel overboard to prevent overpressurizing the tanks, as long as the fuel nozzle pressure is not in excess of 55 psi. A wing fence, located on the wing lower surface inboard of the vent scoop, prevents fuel spillage from traveling inboard along the wing either on the ground or in flight.
5C8 
Aug 01/96  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  28-13-0 Page 1 

2.  Fuel Vent Scoop
A.  The fuel vent scoops, located in the outboard half of the vent surge tanks, are the overboard vents for the vent system. Each scoop consists of a recessed ram air scoop mounted in a lower wing surface access panel, and a five inch standpipe assembly. The diverging walls of the standpipe act as a plenum chamber ensuring that ram air will be applied to the vent system during all flight attitudes of the airplane. A screen is installed at the top of the standpipe to prevent the entrance of large insects, birds, or foreign matter. The design of the scoop is such that icing conditions have minimum effect on ram air pressure.
3.  Vent Surge Tank
A.  A 30-gallon surge tank outboard of each wing tank provides fuel surge capacity for the vent system. The purpose of the surge tank is to accommodate fuel surges during rapid attitude changes of the airplane. This integral tank incorporates a connection to atmosphere through the vent scoop and a drain line connection to the center tank vent duct. Fuel will flow through the vent duct into the center tank (Fig. 1).
4.  Vent System Sealing
A.  The wing tank vent system is sealed in all areas where fuel leakage might occur. Seal applications are designed to prevent external leakage and also to prevent leakage between tank and vent areas. Sealing is accomplished by the use of a sealant compound which is applied around structural fasteners, between faying surfaces, and along the edges of structural members and equipment. All vent system tubing and equipment is electrically bonded to the airframe with bonding jumpers or by faying surface bonding. Sealing application, leak detection and isolation, and sealing repairs for any portion of the vent system require special techniques and procedures (Ref 28-11-0 and 28-14-51, Approved Repairs).
543 
28-13-0 Page 2  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Aug 15/73 

5C8  Fuel Vent System 
Aug 01/96  Figure 1  28-13-0 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

6.  Fuel Vent Float Valve
A.  A fuel vent float valve is used in each fuel tank to prevent a surge of high level fuel from entering the vent duct or tubing when flight attitude is out of level.
B.  Tank No. 1 and 2 vent float valves are located in the outboard vent duct of the wing tanks. The valves consist of a float mounted on an arm hinged to a mounting base. The mounting base is installed over the vent port on the lower surface of the sealed vent duct (Fig. 1). A valve stopper attached to the float arm incorporates a seal which contacts and seals the vent port when the valve is closed. The valve is held in the normally open position by the weight of the float and float arm against a stop attached to the valve mounting base. The stop limits the downward travel of the float. As fuel rises in the tank, the float rises closing the valve aperture. When the fuel level recedes, the valve reopens.
C.  The center tank fuel vent float valve is mounted at a high level in cavity No. 3 to prevent fuel from entering the vent tubing. The valve is similar in function and detail to the tank No. 1 and 2 float valve described above except for the valve seat and stop details (Fig. 1).
　
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