28-11-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
S 796-189
(7)
To find the internal leak source, do one or more of the subsequent procedures.
H.
Air Pressure and Bubble Solution (Backblowing) Method
NOTE: This procedure gives good results for leak detection.
____ S 946-007
(1)
Two persons are necessary to do this task:
(a)
One person must be out of the tank at the external location of the leak.
(b)
One person must go into the tank and find the internal location of the leak. 1) The person that goes into the fuel tank must have these
items: a) An explosion-proof flashlight b) Non-corrosive soap compound, Snoop Leak Detector
(MIL-PRF-25567) or bubble bath solution c) Marking pen (Berol Filmograph or equivalent)
(c) If the external leak location is near a tank end rib, the leak can be in either one of the tanks.
NOTE: It is a faster procedure to put a person in each of the
____ two tanks at the same time to look for the internal leak.
S 946-008
(2)
The two persons must agree on a procedure to communicate.
(a)
The person out of the tank can knock on the airplane skin to make it easier for the other person to find the correct location.
(b)
The person in the fuel tank must tell the person out of the tank when to blow air through the external leak.
(c)
Agree on a system of knocks.
NOTE: For example, the person in the fuel tank can knock one
____ time to start blowing air, two times if he finds a leak, and three times if the leak is not found.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
S 286-010
(3) The person in the tank must apply the soap solution to a large area near the external location of the leak.
NOTE: The internal leak can easily be as far as three feet
____ (1 meter) from the external location of the leak. Examine the internal surface of the tank for loose sealant or bad sealant or a bad fastener. This can help you make a decision about where to look for the internal leak. If the external leak location is a fastener, find the fastener on the internal surface of the tank. Examine this fastener before you try to find the leak in a different location.
(a) If the external leak location is a skin joint (Fig. 605), examine the fillet seal on the nearest splice stringer (Fig. 603, Fig. 604).
NOTE: Splice stringers are stringers that cover the joints of
____ the airplane skin (Fig. 603, 604). There are two splice stringers along the top of the wing tanks: No. 14 and 27. There are four splice stringers along the bottom of the wing tanks: No. 5, 10, 15 and 20. There are four splice stringers in the center tank along the spanwise beams: No. 1, 2 and 3; and the midspar. Faying surface seals ("isolation seals") are applied at intervals of 30 inches along the splice stringers. These seals are applied between the splice stringers and the airplane skin. Their purpose is to keep the length of a leak path along a splice stringer to a limit of approximately 30 inches. Leak paths and backblowing paths do not usually go through these isolation seals.
(b)
If the external leak location is the front or the rear spar, examine the fillet seals on the internal surface of the applicable spar.
(c)
If the external leak location is along a tank end rib, look for internal leaks in the two tanks (Fig. 603, 604).
NOTE: It is a good procedure to put one person on each side of
____ the tank end rib. Thus, one person is in each tank. These two persons must look for internal leaks in the fillet seals on each side of the rib. Examine the full length of all of the fillet seals. It is possible for the head pressure of the fuel in the tank to cause fuel to show at an external location that is higher than the internal leak location.
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