TM 1-1500-204-23-11
well on lat surfaces where there are no creases in
NOTE
the bagging ilm and the leak is suspected to be the
bag and not in the seams. Care should be taken to
The minimum permissible vacuum for com-
watch the edges of the parts where the release ply
posite repairs is 20 inches Hg unless a lower
cannot make full contact with a corner.
vacuum is speciied.
Refer to the system speciic technical manual
(a) Vacuum Bag Sealant Tape Technique. If
for vacuum criteria for your repair procedure
thermocouple wires or heat blanket power cables
to ensure no restrictions exist in performing
contain an outside sheath or insulation in addition to
this test at full vacuum. Delicate compo-
individual wire insulation, the gap between the outer
nents, honeycomb core, and certain enve-
insulation and wire insulation may allow air to leak.
lope-bagged parts could be damaged from
Remove 1 inch of the outside insulation in the area
excessive vacuum pressure if the proper pro-
where the wires interface with the vacuum bag sealant
cedures are not followed.
tape.
(g) When full or the maximum allowed vac-
(b) Frontside Seal Technique. The frontside
uum level is reached, disconnect the
seal technique is used when leak paths are suspected
vacuum source itting from the bag.
around fasteners, joints or substructure. It consists
Observe the vacuum gauge for any
of coating the suspected leak areas, fasteners, etc.,
signs of leakage. Consult the aircraft
with an epoxy adhesive or sealant. Any material used
technical manual for the acceptance
in the sealing process must be able to survive the
criteria for the vacuum bag leak test.
maximum temperatures used in the repair process.
In the absence of speciied criteria,
Follow these steps to perform this technique:
ensure the vacuum bag does not lose
more than 1 in Hg over a 1-minute
Solvent clean the repair area. Pay
1
period. Refer to the system speciic
special attention to any cracks or
technical manual for vacuum criteria
crevices around fasteners and joints.
for your repair procedure to ensure no
restrictions exist in performing this test
2
If removable fasteners are present
at full vacuum. Delicate components,
in the repair area, remove them
honeycomb core, and certain enve-
and reinstall wet with an approved
lope-bagged parts could be damaged
sealant. Refer to the system speciic
from excessive vacuum pressure if the
technical manual for your applica-
proper procedures are not followed.
tion. If blind side fasteners or rivets
are present and backside access is
(h) If unable to reach the required vacuum
possible, sealant or adhesive may be
level or the bag fails to maintain the vac-
applied at the fastener to skin junc-
uum level after the vacuum source is
tion. If a gap or joint is in the repair
disconnected, the repair area must be
area and is a suspected leak path,
sealed before continuing with the repair.
apply sealant or adhesive to the sus-
pect area.
(3) Solving for Vacuum Leaks. Vacuum leaks
If the previous steps do not solve the
3
within the repair area are known to cause porous
leak problem and there is no back-
bondlines and weak repairs. Therefore, all cracks,
side access to the blind fasteners or
fastener holes, and joints in the structure must be
no obvious leak paths are visible, per-
sealed to prevent vacuum leaks and entrapment of
form the following procedure:
processing chemicals. Extreme care should be given
to all bagging ilm; simply throwing the thermocouple
a
Using an appropriate spreader
wires onto the bagging ilm can cause a leak that
or applicator, apply a thin coat
may be very dificult to ind. Keep all sharp edges
of epoxy adhesive or sealant
away from the bagging ilm and protect the bagging
over the entire repair area; place
ilm from creases and folds. Several methods can
an approved peel ply over the
be used to solve vacuum leaks. One method is with
resin and cure it per the manu-
the use of leak detectors. These instruments can be
facturer s directions.
very expensive and elaborate and come in all shapes
and sizes. Most of the time the vacuum leaks can be
b
Once the sealant has cured,
detected using your ear. Another method is to drag
carefully remove the peel ply
a thin ilm of non-porous release ilm across the bag
and sand the repair area until all
and look for dimples in the ilm. This method works
6-66
