(b)
If it is possible, install one of the pressure cups supplied in the HeliTest Wing Kit to contain the pressure of helium on the outer surface of the tank.
S 796-338
(7)
Install tape, SM5126 or SM5127 around the leak area or the pressure cup.
(a)
Work the tape to stick it to the surface.
S 796-339
(8)
If you build a custom chamber, do these steps:
(a)
Cut a piece of material, L-P-512 to the size of the chamber formed by the tape, SM5126 or SM5127.
(b)
Set the plastic over the vacuum tape.
(c)
Work the plastic to make sure it sticks well to the vacuum tape.
(d)
Make sure the chamber is as small as possible.
NOTE: A small chamber can contain more helium pressure than a
____
larger one.
(e) Trim the plastic with a razor knife to follow the outline of the vacuum tape.
S 796-340
(9)
Do these steps to connect the helium supply to the custom chamber:
(a)
Wrap the helium injection tube with vacuum tape to form a 2-inch (5 cm) diameter flange 1 to 2 inches (2.5 to 5 cm) from the end of the tube.
(b)
Slice a small hole in the plastic chamber and put the helium tube in the hole.
(c)
Work the vacuum tape flange on the injection tube so it sticks well and seals the plastic chamber.
S 796-341
(10)
If you use a pressure cup, attach the helium injection tube to the cup with the fittings supplied with the kit.
S 796-342
(11)
Apply tape, 3M #434 or equivalent, over the plastic chamber or pressure cup and work it well for the best adhesion.
(a)
Use a sufficiently large quantity of tape, 3M #434 to reinforce the plastic helium chamber so it can handle more pressure.
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MAINTENANCE MANUAL
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(12) Set up the HeliTest Wing Kit and purge the lines with helium to remove air.
NOTE: Refer to the instructions supplied with the kit.
____
S 796-344
(13) Carefully and slowly apply about 0.2 psig helium pressure to the chamber.
NOTE: Start with low pressure and then slowly increase the
____ pressure. Less helium is necessary for larger leaks than for smaller leaks. Too much helium will flood the fuel tank. Inspect the chamber or rubber cup to make sure there is no leakage. If it leaks too much, rebuild the chamber because the helium can interfere with the detector.
S 796-345
(14) Put negative ventilation around the helium chamber to draw away leaking helium to make sure it does not interfere with the detector by drifting into the tank.
NOTE:____This step is important for successful leak detection. Use positive pressure if negative pressure is not available, but make sure the helium is removed from the area and does not go into the tank where the detector probe is used.
S 796-346
(15) Permit the helium to soak into the leak path for 10 to 15 minutes before you go into the tank.
NOTE: Time is necessary to push the remaining fuel out of the tank
____ path and start the flow of helium from the external leak point to the internal leak point.
S 796-347
WARNING:_______KEEP THE DETECTOR AND THE PUMP AWAY FROM THE TANK. INJURY TO PERSONS AND DAMAGE TO EQUIPMENT CAN OCCUR.
(16)
Keep the HeliTest detector and the auxiliary pump away from the open fuel tank at all times.
S 796-348
(17)
Go into the tank with the HeliTest Visual Probe (intrinsically safe for Class I, Div I areas).
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
28-11-00
ALL ú ú 05 Page 628 ú Oct 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
S 796-349
(18) Slowly interrogate the inside of the tank around the area where you expect to find the leak.
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