(f)  
The person in the tank must tell the person out of the tank when to blow air through the external leak.

NOTE:  Agree on a system of knocks. For example, the person in the tank knocks one time to start the backblowing, two times if he finds the leak, three times if the leak is not found.

(g)  
The person out of the tank must use an air supply with a nozzle to blow air through the external leak location (Fig. 101).

NOTE:  The external leak location can be a hole in the aerodynamic smoother, a bad fastener, or an open seam in one of the spars, or the side-of-body rib.
1) If the leak is small, it can be necessary to use the maximum pressure (100 psi). 2) If the leak is larger, it is easier to find the leak with less pressure (for example,
50 psi or less).
3) Look for bubbles that show the location of the leak.
4) It is also possible to feel for blowing air to find the leak.
5) Find all leak locations on the internal surface of the fuel the tank.

NOTE:  It is possible that three or four internal leaks are related to one external leak location. Make a signal to the person out of the tank to stop the backblowing procedure when you find all of the leaks.
Feb 01/96 28-11-0 Page 104C/104D BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

6)  Make a mark on these leak locations with the marking pen.
NOTE:  If you are not sure you found the correct leak path, you can apply the bubble solution to the external leak location and blow air through the internal leaks. If air blows through the external leak location, you found the correct leak path.
(h)  
If you find a leak in a self-sealing fastener, refer to 28-11-0/801 for procedures to do a temporary repair of self-sealing fasteners with leaks.

NOTE:  Self-sealing fasteners are frequently used in the fuel tank. These fasteners have a plastic collar that prevents fuel flow. They are not covered with sealant. Do not put sealant on these fasteners unless you find one with a leak. The only permanent repair of a self-sealing fastener with a leak is to remove the used fastener and install a new one (Ref Structural Repair Manual).

(i)  
Remove soap solution from in tank.

500  Leak Detection Air Hose and Bubble Method 
Feb 01/96  Figure 101 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  28-11-0 Page 105 

J.  Pressure Box Application
(1)  To locate small leaks which require application of air pressure for longer periods of time, a pressure box may be fabricated. (See figure 102).
CAUTION: MAXIMUM PRESSURE ALLOWABLE IN THE BOX IS 4 PSIG.
THE AIRPLANE WING MUST NOT BE LOWERED BY ANY MEANS WHILE BOX IS IN POSITION. EXAMPLE: FUEL TANKS MUST NOT BE REFUELED OR TIRE PRESSURES REDUCED.
EXCESSIVE JACKINGUP OF BOX MUST BE AVOIDED.
APPLY AIR PRESSURE TO BOX SIMULTANEOUSLY WITH APPLICATION OF LOAD TO THE BOX TO PREVENT DAMAGE TO WING PANEL.

Pressure Box Application  500 
28-11-0  Figure 102  Nov 15/68 
Page 106 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

K.  Dye Injection Method
(1)  
This method consists on injecting dyed fuel through the external leak point to the leak source inside the tank by use of a leak tracing device (Fig. 103). Use nonfluorescent red dye in concentrations of up to 2 fluid ounces per 100 gallons of fuel.

(2)  
Intense green or bluish white fluorescent dye mixed 1-ounce dye to 1-gallon fuel, and an ultraviolet light, may also be used for this method of leak detection.

(3)  
With tank prepared for entry per 28-10-0, Maintenance Practices, and leakage tracing device F71329 located as close as practical to test area, proceed as follows:

(a)  
Follow instructions in lid of leakage tracing device for use of items in the box. If a substitute tool is used follow applicable instructions in order to obtain the desired results.

(b)  
After complying with all tank safety precautions (Ref 28-10-0 MP), enter tank and observe suspected leak area for indication of dye.

CAUTION:  MONITOR DYE CONTAINER GAUGE AND EQUIPMENT WHILE IT IS IN POSITION.

(c)  
Continue dye pressure application, as long as required, to allow dyed fuel to travel leak path.

(d)  
When dye appears inside tank, mark point of appearance.

(e)  
Trace possible leak path or paths and locate possible seal plane penetration. A sketch of possible leak paths should be made to aid in coordinating sealing repair and prevent the possibility of overlooking an isolation, or prepack seal in the structure.
　
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