E. Check locations where injection seals have been installed to provide continuity for fillet seals.
F. Look for channels which might have been bridged by sealant rather than filled.
G. Check tank structure for cracks or distortion.
H. Visible defect in sealing or structure is not necessarily origin of fuel leak. Any defect should be repaired to prevent subsequent deterioration and fuel leakage. If the leak source has not been definitely located by visual inspection, one of the following methods should be used.
I. Backblowing Procedure to Find the Internal Leaks
(1) If leak source has not been definitely isolated by a visual check, it may be revealed by the application of a soap solution to inside of tank.
(a)
Prepare the tank that has the leak for entry as described in Fuel Storage System Maintenance Practices, 28-10-0.
(b)
Two persons are necessary to do this task: 1) One person must be out of the tank at the external location of the leak. 2) One person must go into the tank and find the internal location of the leak. 3) If the external leak location is near the side-of-body rib, the leak can be in the
center tank or the wing tank.
NOTE: It is a faster procedure to put a person in the center tank and in the wing tank at the same time to look for the internal leak.
(c)
The two persons must agree on a procedure to communicate.
(d)
One person must go into the tank to a location near the external leak location.
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Feb 01/96 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 28-11-0 Page 104A
NOTE: 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.
1) The person that goes into the tank must have these items:
a) An explosion proof flashlight
b) Noncorrosive soap solution (bubble bath)
c) Marking pen (Berol Filmograph or equivalent)
(e) 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 from the external location of the leak that you found before. 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.
1) If the external leak location is a skin joint, examine the fillet seal on the nearest splice stringer.
NOTE: Splice stringers are stringers that cover the joints of the airplane skin. 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.
2) If the external leak location is the front or the rear spar, examine the fillet seals on the internal surface of that spar,
3) If the external leak location - the boundary between the wing tank and the center tank, look for leaks in these two tanks.
NOTE: It is a good procedure to put one person in the wing tank and one person in the center 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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