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MAINTENANCE MANUAL
(2)
Faying Surface Seals (Fig. 10)
(a)
Faying surface seals perform several functions in fuel tank sealing. As primary seals they are used between surfaces where an absolute seal cannot be economically obtained by the use of sealed fasteners and fillets. A faying surface seal around a fastener prevents the fuel from reaching the fastener area. It does not entirely seal the fastener which must also be sealed. Faying surface seals are especially used to seal areas that will be hard or impossible to seal after assembly. Faying surface seals are also frequently used as isolation seals between the front and rear spar chords and wing skin. They are spaced along the spar chord. Therefore a leak along the spar chord can be isolated between the seal that had stopped the flow of fuel and forced it into view and the next isolating seal upstream. Isolation seals are found only in dry areas, that is, areas beneath structure where primary sealing is designed to prevent fuel from flowing.
WING RIB
PRIMARY SEALS
FAYING SURFACE SEAL BETWEEN RIB ATTACHMENT ANGLE AND SPAR WEB
RIB ATTACHMENT ANGLE
SPAR WEB SPAR WEB
ISOLATION SEALS
Faying Surface Seal
Figure 10
SEALED
FASTENERS
FILLET SEALS
FAYING SURFACE SEAL AROUND FASTENERS
ISOLATION SEALS
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(3)
Injection Seals (Fig. 11)
(a)
Holes, joggles, channels, and other voids caused by buildup of structure in the fuel tank boundaries are normally filled by injection seals. Injection seals may be applied to provide continuity where fillet seals are interrupted by structure. Holes, joggles, and channels along seams that are to be fillet sealed are also injected full of sealant to provide a backup seal to support primary fillet seal. Voids and channels of large cross sections are packed with caulking (the process of plugging a hole or channel with fiberglass cloth impregnated with sealant) to provide additional strength to the seal.
(4)
Prepack Seals
(a)
Prepack seals are used to fill voids in the tank structure which cannot be reached by the injection method. These voids are packed with sealant before they are closed by the assembly of structural members.
____NOTE: TO PROVIDE CONTINUITY OF SEALANT. SEAL ON BOTH SIDES OF STIFFENER INJECTION SEAL JOINS FILLET
Figure 11 Injection Seals
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(5)
Self-Sealing Rivets (Fig. 12)
(a)
Aluminum alloy rivets are used extensively in fuel tank structure. These rivets are considered self-sealing only if correctly installed. The hole clearance is critical and close tolerance rivets are installed in close tolerance holes. The orientation of rivets and their position with respect to the seal plane is also critical. Detail A illustrates a typical installation of a self-sealing rivet. This rivet is upset into a countersunk close tolerance hole. To seal, the rivet must be driven high, which means that the upset head is slightly larger than the countersunk portion of the hole. The portion of the countersunk head which protrudes above the skin is then removed with a rivet shaver. Actual sealing of the rivet takes place around the bucked head at the bottom of the countersink. This is important in determining how the rivet should be oriented on the seal plane. Countersunk or round head rivets installed in a close tolerance hole and bucked in the normal manner are also considered self-sealing. Sealing of the rivet takes place around the shank just under the bucked head as shown in detail B.
MANUFACTURED
STRINGER
HEAD OF RIVET
STIFFENER
