(2)  
The sump drain valve, used for the integral center fuel tank, is a spring-loaded closed, flapper-type check valve mounted in the wind center section lower skin. A sump drain assembly connects the valve upper housing with the lower body section of the airplane through the keel beam area. The sump drain assembly contains a cam-lock plunger to lift the flapper valve off its seat and allow drainage. To drain the fuel sump, open the hinged access door in the lower body section stenciled SUMP DRAIN, insert a broad-bladed screwdriver in slot, and turn and push up on plunger. To close, release plunger, allow slot to align fore and aft, and secure access door.

539 
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539  Fuel Sump Drain Valve 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

Fuel Sump Drain Valve  539 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

7.  Fuel Tank Sealing
A.  General
(1)  
Integral fuel tank sealing is the means by which the structural members of the wing can be made to form a fuel-tight tank. This is accomplished through the use of wing skin, spars, ribs, stiffeners, and doublers as the periphery of the tanks, and applying sealing materials at areas 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. Refer to Chapter 57 for structural details.

(2)  
Fuel tanks use the sealed structure of the wing to maintain a fuel boundary. This is accomplished primarily by close metal to metal fit of all parts, by sealed fasteners, and by the application of sealing compound to the structure. Optimum sealing is made possible by the use of open section stiffeners throughout the wing which eliminate blind areas. Fillet sealing is used at the spars, fuel vent ducts, and skin splice stiffeners. Rivet seals are achieved by control and orientation of the driven head.

(3)  
Assembly of spar webs, chords, and rib end attachments permit fillet sealing on the tank side. All fillets are on-pressure side seals and are located at areas of minimum deflection. Fasteners installed through the tank end ribs are double sealed. This is because pressure develops on either side of the tank at different times. Bolts or lockbolts are used in high load areas and are sealed with seal cups and filleting materials.

(4)  
Molded rubber seals are used to seal fuel tank access panels. O-rings are used to seal removable plumbing fittings, primary tension bolts, and fuel system components which penetrate the fuel tank walls.

B.  Definition of Sealing Terms:
(1)  
ABSOLUTE SEAL – A level of sealing which requires that all seams, joggles, slots, holes, and fasteners that pass through the seal plane be sealed.

(2)  
CAULKING -- The process of plugging a hole or channel with fiberglass cloth impregnated with sealant.

(3)  
BACKUP SEAL -- A secondary seal which offers support to a primary seal.

(4)  
BASE COMPOUND -- The major component of a sealing material used in conjunction with corresponding curing compound to produce a fuel resistant sealant.

(5)  
CURING COMPOUND (ACCELERATOR) -- Internal curing agent for sealants.

(6)  
EMERGENCY SEAL -- A seal which backs up a primary seal so as to become functional when primary seal failure occurs and safely controls leakage.

(7)  
FAYING SURFACE SEAL -- A seal effected by the sandwiching of sealant between mating surfaces of assemblies.

(8)  
FIRST FILLET -- A small bead of sealant applied over the precoat to ensure complete contact and cover seams or joints.

(9)  
FULL BODIED FILLET -- A combination of first and second fillet.

(10)
INJECTION -- Filling of holes or channels by forcing sealant into a void or cavity.

(11)
ISOLATION SEAL -- A seal placed for the purpose of confining and tracing fuel leakage.

(12)
PREASSEMBLY SEAL -- Sealant material applied during assembly of structure, such as a faying surface or prepack seal.

(13)
PRECOAT -- A thin layer of brushable sealant applied on the surface to be sealed for improving adhesion qualities of the first fillet.

(14)
PREPACKED SEAL -- A quantity of sealant applied prior to assembly which fills voids and cavities.

(15)
PRIMARY SEAL -- Applied sealant directly in line of contact with fuel.
　
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