Faying Surface Seals  500 
28-11-0  Figure 10  May 15/68 
Page 16 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

500  Injection Seals 
Feb 15/80  Figure 11  28-11-0 
Page 17 
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 Rivet Sealing  500 
28-11-0  Figure 12  Aug 15/68 
Page 18 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(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. Where bolts that protrude through the tank wall are used to secure removable components to the outside surface of the tank wall, self-sealing nutplate installations are used to provide fastener sealing. The nutplate installation consists of a dome type, O-ring sealed, nutplate installed with O-ring sealed rivets installed in a single thickness of material. Fuel sump drain valves and fuel quantity measuring sticks are installed in fittings attached to the wing lower skin in each fuel tank. The fittings are attached to the wing 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 wing skin.

500  Lockbolt Sealing 
Nov 15/68  Figure 13  28-11-0 
Page 19 
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Fuel Tank Removable Component Seals  500 
28-11-0  Figure 14  May 15/68 
Page 20 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

8.  Integral 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 will flow in all directions, sealing is made a problem involving three dimensions. Figure 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 wing skin. The seal plane is maintained by a continuous fillet along the entire length of the stringer and in the area of the lockbolts.

500  Typical Fillet Seal Leak 
May 15/68  Figure 15  28-11-0 
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B.  Isolation seals assist in locating leaks by limiting the distance fuel can flow between structural members. Figure 16 illustrates a typical leak situation on a wing 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 figure 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 Effect of Isolation Seal  500 
28-11-0  Figure 16  May 15/68 
Page 22 
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9.  Leak Evaluation and Re-pair Requirements
A.  Fuel leakage from a tank is divided into four groups to provide a means of evaluating the safety of flying the airplane with fuel leaks present. The four groups are stain, seep, heavy seep, and running leak. Each group is determined by a visual examination of wetted area around the leak source. Wetted areas must be wiped clean after each examination for accurate re-evaluation of leak. The size patterns of leaks illustrated in figures 17 and 18 are based on an examination 15 minutes after leak was first noticed and area wiped clean. In figure 17 the four groups of leaks are evaluated as follows:
　
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