(a)
Synthetic rubber makes the base for most sealing compounds.
NOTE: The manufacturer adds ingredients to control strength,
____ cure time, plasticity, usable life, and fuel resistance.
(b) Sealants are supplied in two parts; base material and
accelerator.
NOTE: You must be very careful to make sure the correct
____ proportions of the base material and the accelerator recommended by the manufacturer are used. If you do not obey the manufacturer's instructions, you can change the physical properties of the mixture which can cause a seal failure and a fuel tank leak.
(c) Sealing compounds have a specified shelf life.
NOTE: After the specified time, you must do a test of the
____ sealing compounds to find out if you can use them.
(d) You can keep some sealants in refrigeration for a short time after they are mixed with the accelerator.
NOTE: You must discard these sealants after the specified
____
time.
S 366-263
(3)
Sealants are applied incorrectly (AMM 28-11-00/801).
(a)
If you do not do the subsequent steps, fuel leaks can occur: 1) Follow all manufacturer's instructions. 2) Use a brush to apply precoat when recommended. 3) Do all the steps to apply the sealant. 4) Make sure there are no air bubbles in the sealant. 5) Make sure you fill all the spaces completely with sealant. 6) Make sure you do not make an overlap with the sealants. 7) Make sure the sealant touches all the surfaces.
S 366-264
(4)
The fasteners are loose.
(a)
Loose fasteners start fuel leaks because they let attached surfaces move.
(b)
Faying surface seals get cracks and let fuel leak through the seal plane (AMM 28-11-00/801).
(c)
Loose rivets are not self-sealing.
(d)
Sealant or metal seal covers do not bond with loose fasteners.
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A
747-400 MAINTENANCE MANUAL
S 366-265
(5)
Fittings and connections are installed incorrectly.
(a)
Fittings and connections attached to or that go through the fuel tank structure are sealed with O-rings.
NOTE: Damaged or incorrectly installed O-rings can cause fuel
____
leaks.
E. Leak Path Analysis
S 796-273
(1)
Find the external leak point (Fig. 603, 604).
S 796-274
(2)
Find the internal leak source.
S 796-275
(3)
Find the point where fuel goes through the seal plane.
(a)
The internal area where you see the bubble or dye usually shows the point where fuel goes through the seal plane.
(b)
If there is an injection, prepack or hidden seal failure, the fuel can move along the tank structure and leak at a point far from the leak source.
(c)
If you repair the internal area where you see the bubble or dye and do not repair the injection, prepack or hidden seal failure, you can only temporarily repair the leak.
(d)
You must find all possible leak paths between the external leak point and internal leak source to repair the seal failure.
NOTE: You can raise the seal plane as an alternative to a
____ repair of the seal (AMM 28-11-00/801).
S 366-267
(4)
The seal plane stops the fuel leaks from the fuel tank.
(a)
When there is a seal plane failure, the mating surfaces of the fuel tank structure, external to the seal plane, get wet.
NOTE: The area that gets wet extends in all directions from
____ the point fuel goes through until it finds an injection, faying surface, prepack or hidden seal. The seal plane stops the release of fuel from the fuel tank.
(b) There are no seals on the external side of the seal plane and fuel that goes through the seal plane follows the path of least resistance. 1) For example, a row of fasteners in a groove are wet but
only one fastener has a leak. 2) If the fastener with the leak is sealed, then the fastener with the next least resistance leaks.
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