Fuel Tank Access Doors
Figure 2 (Sheet 2)

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747-400
MAINTENANCE MANUAL

HIGH IMPACT 
RESISTANT 
ACCESS DOOR 
(7 PLACES)  STANDARD
SEE  C  ACCESS DOOR  STANDARD 
(2 PLACES)  ACCESS DOOR 
SEE  B  (10 PLACES) 
SEE  C  SURGE TANK 
ACCESS DOOR 
SEE  D 
NO. 1 
MAIN TANK  NO. 2  SURGE 
RESERVE TANK  TANK 
PRESSURE 
RELIEF 
VALVE 

ACCESS DOOR
D
Fuel Tank Access Doors Figure 2 (Sheet 3)
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747-400
MAINTENANCE MANUAL

 D.  The valve is designed to allow the primary poppet to be removed without draining the fuel tanks. When the seat for the primary poppet is removed, the secondary poppet closes, preventing fuel from flowing from the tank. During installation, the secondary poppet will not lift off its seat until the primary poppet seat is positively engaged.
 5.  Baffle Check Valves (Fig. 4)
___________________
 A.  Baffle check valves are installed in various ribs in the main fuel tanks to reduce the rate of fuel flow away from the fuel feed boost pump inlets during airplane pitch and roll attitude changes. Each check valve consists mainly of a flapper which is hinged at the top to open in one direction only. The flapper is free-swinging and aligns itself automatically with the valve seat when it closes. Most check valves are installed to allow fuel flow in the inboard/forward direction only. The four check valves installed in a baffle between adjacent ribs in No. 2 and No. 3 main tanks provide additional aid in ensuring that fuel will be held in the inboard/forward portion of the tank especially during steep climb attitudes with low fuel tank quantities.
 6.  Overwing Fill Ports (Fig. 5)
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 A.  Overwing fill ports are located in the four main tanks only. Each fill port consists of a filler cap adapter, a seal ring and clamping nut fitted into a hole in the upper wing panel. A flush fitting cap assembly is installed in each fill port and locked in place by rotating the cap clockwise to a stop. This compresses an O-ring in the cap against the inside of the adapter to seal the fill port opening.
 7.  Center Wing Tank Purge Door (Fig. 6)
___________________________
 A.  The center wing tank purge door is located in the rear spar in the right wing gear wheel well. The door is used to ventilate the tank during purging operation. Also, the door opening is used to attach a special door when applying air pressure to leak test the tank. The purge door consists of self-contained nutplates, an O-ring seal, and a means of attaching a lanyard to retain the door when the mounting bolts have been removed.
 8.  Fuel Tank Sealing
_________________
 A.  General
 (1)  Integral fuel tank sealing allows wing structure to be used to form a fuel-tight tank. Skin, spars, ribs, stiffeners, stringers, and doublers form the periphery of the tanks, and sealing materials are applied where these members join. Sealing materials are generally synthetic rubber, either in molded form (O-rings, gaskets, etc.), or in an uncured state (for brush or gun application). Use of sealant is held to a minimum to avoid excess weight.
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747-400 MAINTENANCE MANUAL
CENTER WING NO. 2 RESERVE TANK

WING POPPET SKIN (REF)
A-A
Fuel Sump Drain Valves
　
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