D. Types of Seals (See figure 8.)
(1)
Emergency Seals
(a) Emergency seals are used as a backup seal to direct flow of fuel to an external drain in the event of a primary seal failure. The removable fuel cells (when installed) in the wing center section are considered primary seals.
(2)
Fillet Seal
(a) The basic sealant application to fuel tank structure is the fillet seal. Fillet seals are used to cover structural joints along stiffeners, stringers, tank walls, and wing spars, and to seal around fittings on tank walls and in corners. The dimensions shown in figure 9 represent typical finished fillet shapes for various gages. Fillets conform to the dimensions as nearly as possible since the shape of a fillet determines how long it will last in service. A typical application of fillet seals in a tank corner is also shown in figure 9.
(3)
Faying Surface Seals (See Figure 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 will be found only in dry areas, that is, areas beneath structure where primary sealing is designed to prevent fuel from flowing.
(4)
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 which are to be fillet sealed are also injected full of sealant to provide a backup seal to support the 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.
(5)
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.
(6)
Self-Sealing Rivets (See figure 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.
(7)
Fastener Seals (See Figure 13.)
(a) Since lockbolts are not self-sealing, sealant must be applied to prevent leakage. The most common application of sealant is to the end of the fastener on the “wet” side of the tank structure. Current practice dictates that fillet seals and seal caps be used. Repairs to leaking lockbolts should conform to this method.
(8)
O-ring Seals (See figure 14.)
500 Integral Fuel Tank Seals
Nov 15/68 Figure 8 (Sheet 1) 28-11-0
Page 13
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
Integral Fuel Tank Seals 500
28-11-0 Figure 8 (Sheet 2) May 15/68
Page 14
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
500 Fuel Tank Fillet Seals
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