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737-300/400/500MAINTENANCE MANUAL
 (2)  The seal plane is the barrier that keeps the fuel in the fuel tank. When the seal plane has a hole, the faying surfaces of all the structure on the other side of the seal plane become wet. The wet area becomes larger in all directions from the hole in the seal plane until it touches an injection, faying surface, or prepack ("hidden") seal. Fuel leakage usually flows along fillet seals until it gets to a point where it can get out of the structure (the external leak point). Because there are no seals on the dry side of a seal plane, the fuel will flow out of the structure at the point with minimum resistance to fuel. For example, a row of fasteners in a leak path are wet, but the leak is shown at only one fastener. If you seal the fastener with the leak, then the remaining fastener with the minimum resistance to fuel will have a leak. This process will occur again until you repair the seal surface. Make an analysis of the leak path to find out how the fuel went from the hole in the seal plane to the external leak point. It is necessary to know the tank structure and the sealant to know the leak path fully.
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 6.  Fuel Leak Detection and Repair Aids___________________________________
 A. General
 (1)  
This procedure has these tasks:

 (a)  
Leak Detection Tips

 (b)  
Leak Repair Tips

 (2)  
Leak tips are given for these parts of the fuel tank:

 (a)  
No. 1 or No. 2 Internal Tank - Forward Inboard Upper Corner

 (b)  
Center Tank - Forward Outboard Upper Corner

 (c)  
External Rear Spar Upper Splice

 (d)  
No. 1 or No. 2 Internal Tank - Aft Inboard Upper Corner

 (e)  
Center Tank - Aft Outboard Upper Corner

 (f)  
No. 1 or No. 2 Internal Tank - Forward Inboard Lower Corner

 (g)  
Center Tank - Forward Outboard Lower Corner

 (h)  
External Rear Spar Lower Splice

 (i)  
No. 1 or No. 2 Internal Tank - Aft Inboard Lower Corner

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737-300/400/500MAINTENANCE MANUAL
REAR SPAR
ACCESS PANEL STRINGER
IN THE WING NO. 5
ROOT RIB
STRINGER NO. 9
INBD
 FWD
WING ROOT RIB (THE SIDE IN THE NO. 1 TANK)
A
REAR SPAR
ACCESS PANEL IN THE WING ROOT RIB
INBD
 FWD
STRINGER NO. 5
(SKIN JOINT)

STRINGER NO. 9
(SKIN JOINT)

LEGEND
WING ROOT RIB
THE HEAVY LINE
(THE SIDE IN THE CENTER TANK)
SHOWS SOME OF THE
FILLET SEALS ON

B
THESE SURFACES
Seal Surfaces of the Integral Fuel Tanks
Figure 601 (Sheet 2)

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A 737-300/400/500MAINTENANCE MANUAL

HEAVY SEEP  RUNNING LEAK

Fuel Leak Patterns
Figure 602

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