(16)
RAISED SEAL PLANE -- A new seal plane established at a point farther from the tank exterior than the original seal plane to repair a preassembly seal.

(17)
RE-ENTRANT FILLET EDGE -- An overlapping quantity of sealant not in close contact with adjacent sealant or structure which is a potential leak source through the seal plane.

(18)
REPAIR SEAL -- A seal placed for the purpose of limiting the amount of sealing required to repair a fuel leak.

(19)
SEALANT -- Mixture of base compound and curing compound (accelerator) which will cure to a firm fuel-resistant material.

(20)
SEAL PLANE -- The interior surfaces of the tank, comprised of structure, sealed fasteners, and sealant, which actually contain the fuel.

(21)
SEALANT STOP BOLT -- Bolt installed through an injection seal to provide support for a seal.

(22)
SECOND FILLET -- A bead of sealant applied over the first fillet.

(23)
SECONDARY SEAL -- Sealant applied behind and in conjunction with a primary seal to confine fuel.

(24)
SQUEEZE-OUT LIFE -- Length of time sealant remains suitable for structure assembly in faying surface seal application.

(25)
TACK-FREE TIME -- That point of time in the curing of a sealant or a topcoat at which the material does not stick to a dry knuckle pressed lightly against the sealant or topcoat and withdrawn.

(26)
TOPCOAT -- A continuous film which offers corrosion protection to metal surfaces and general protection for sealant against bacteria present in hydrocarbon fuels.

(27)
WET-SIDE SEALING -- Sealing which is accomplished by applying sealant to fasteners or seams on the fuel side (wet side or inside) of a fuel tank.

(28)
FAIRING TOOL -- A tool used to form a sealant fillet. Any clean tool that will achieve an acceptable fillet may be used.

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C.  Typical Example of Seal Planes (Fig. 7)
(1)  The basic principle of fuel tank sealing is the establishment of a seal plane. The seal plane encompasses the tank structure and sealed components beyond which fuel is not permitted. Typical examples of seal planes are as follows:
(a)  
The most common example of using fasteners to obtain the seal plane is in the stringer to skin application shown in detail A. The fuel is held in the tank by the metal to metal fit of the rivet head upset into a countersunk hole. Leaks in these areas are caused by leaking fasteners. This principle is also used in rib attachments to the spars as shown in detail B.

(b)  
In areas where the skin is spliced, the seal plane cannot be maintained by fasteners alone, because fuel would still flow under the stringer and leak out at the splice. To prevent this the seal plane is raised by the addition of fillet seals along the stringer as shown in detail C. Leaks in this area would be caused by a breakdown of the fillet seal.

(c)  
In areas where several parts axe fastened together, such as a skin splice at a stringer, the seal plane is kept close to the skin by the use of prepack seals. In this case, the prepacked seal is applied at the chamfered corners of the angles during assembly as shown in detail D. This eliminates fillet sealing along the edges of the vertical legs of the angles and around the non-self-sealing rivets. Note that failure of any of the seals would result in fuel leakage between mating surfaces and out through the skin splice or out between the fastener shank and the hole.

(d)  
An absolute seal is required to isolate the spanwise vent ducts from all surrounding areas. The ducts must be sealed against external leakage and also against leakage between the vent system and the fuel tanks. Detail E, shows a typical vent duct seal plane application. The enlarged view shows that the seal plane is maintained by self-sealing rivets and by sealant. The upset head of the rivets prevent fuel in the vent duct from seeping out. Fillet sealing along the edges of the vent duct prevents fuel from entering the duct through these two points. Fuel in the vents is also prevented from leaking back into the tank.

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Fuel Tank Seal Plane Details  500 
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