Rivet Sealing
Figure 12
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(6)
Fastener Seals (Fig. 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. Fig. 13 illustrates application of sealant to fastener ends. Current practice is the use of fillet seals and seal caps. Repairs to leaking lockbolts should conform to this method.
SEAL CAP
Lockbolt Sealing
Figure 13
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(7)
O-ring Seals (Fig. 14)
(a)
The type of sealing used for removable components which penetrate the fuel tanks is determined by their use in the tank structure. Bolts which carry primary tension loads are sealed with O-rings installed under special washers. Bolts that require removal in service are sealed with standard seal washers. Other bolts are sealed by applying seal caps or fillets of sealant to the end of the bolt on the wet side of the tank. Fuel sump drain valves and fuel quantity measuring sticks are installed in fittings attached to the wing lower tank skin in each fuel tank. The fittings are attached to the skin by self-sealing rivets and the fittings are sealed by a fillet seal around their periphery. Sealing between the drain valves or measuring sticks and their fittings is accomplished by installing O-rings during installation. Bulkhead fittings in the fuel lines are sealed with O-rings in a similar manner. Fuel tank access panels are sealed by a molded rubber seal ring between the panel and the tank skin.
TENSION BOLT
RECESSED WASHER
Figure 14
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9. Fuel Tank Leak Detection
________________________
A. Fuel has a very low surface tension and viscosity; therefore, it is able to penetrate the slightest flaw in the seal plane. Because fuel under pressure flows in all directions, sealing is made a problem involving three dimensions. Fig. 15 illustrates a typical fuel leak situation in the seal plane. Fuel flowing under the sealed stringer will eventually reach a point where it can penetrate the tank skin. The seal plane is maintained by a continuous fillet along the entire length of stringer and in the area of the lockbolts.
B. Isolation seals assist in locating leaks by limiting the distance fuel can flow between structural members. Fig. 16 illustrates a typical leak situation on a spar. Fuel from this type of leak has been found to travel in the spar area instead of becoming visible near the point of seal plane penetration. However, when the leaking fuel reaches an isolation seal, it is forced outwards and becomes visible. In this case the leak path is confined to the area between the two isolation seals shown in Fig. 16. It is not necessary to know the location of these seals. Knowing the location of the seals will, however, greatly assist in the location of seal plane penetration.
Typical Fillet Seal Leak
Figure 15
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A 747-400 MAINTENANCE MANUAL
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