TM 1-1520-240-BD
(c)
Stiffeners are not permitted in areas
where they will interfere with installation and function or
operation of any component.
(3)
Determine locations where a minimum
number of stiffeners will correct oilcanning or loose skin.
(4)
Add stiffeners as required. Stiffeners
must be attached at each end to primary structure, such
as longeron to longeron or frame to frame. The method
of attachment should eliminate discrepancy without
transferring it to another area. Comply with following
methods:
(a)
Stiffeners must be next heavier
thickness than skin to which they are attached and are
to be formed from 2024-T3 cad aluminum alloy.
Stiffeners exceeding 30 inches in length must be made
from Alcoa 33372 or equivalent extrusions.
(b)
If attaching clips are used, make
from 2024-T2 dad aluminum alloy. They must be same
thickness as stiffener to which they are attached.
(c)
Install
rivets
MS20470AD4
at
approximately 3/4-inch pitch for attachment of stiffeners,
not thicker than 0.040 inch.
(d)
Install
rivets
MS20470AD5
at
approximately 7/8-inch pitch for attachment of stiffeners
thicker than 0.040 inch.
(e)
Seal and refinish stiffeners and clips
to match adjacent structure.
CAUTION
Substitution of materials can create a
corrosion hazard. If the substitute
material is different than original part
(as
when
a
7075-T6
aluminum
extrusion is reinforced with a 4130
steel strap), failure to insulate metals
from direct contact with each other
will permit electrolytic corrosion to
attack repaired area. Insulation of
dissimilar
metals,
is
extremely
important.
b.
Material Substitution. If repair cannot be made
from the same material as original part, select a
substitute material. Refer to Table 2-6 for a list of
substitute materials and an example of how to use this
table. When material for a non-load-bearing part, such
as a filler, is not available in thickness required, build up
to
desired
thickness
with
two
or
more
pieces.
Substitutions
of
this
type
are
not
permitted-for
loadbearing
parts,
such
as
reinforcements
and
insertions.
2-17. SKIN AND WEB REPAIRS. (See Figure 2-34.)
Subsequent
paragraphs
contain
procedures
and
limitations for coin patch repair for skins, backing plate
repair for webs. (See TM 1-1500-204-23 for rivet type
and spacing.) Maintain a minimum clearance of 5/8
inch between cutouts and edge of replacements, unless
otherwise noted. Insulate dissimilar metals.
a.
Coin Patch Repair for Skins. This repair is for
dents, punctures, tears, and small cracks that can be cut
out from 1/2 inch to 1 1/2-inch diameter. No more than
two patches are permitted for each skin panel. Panels,
not less than 15 by 50 inches, between longer longerons
and sta 160 and 482, may contain a maximum of four
patches. All patches must clear adjacent structural
members by 1/8 inch and be sealed.
(1)
Cut out damage and make a coin patch of
same material and thickness as skin. The patch should
have a diameter 5/16 inch less than cutout.
(2)
Make a circular backing plate three times
diameter of coin patch and from same material but in
next heavier thickness.
(3)
Smooth edges of cutout, coin patch, and
backing plate with a file.
(4)
Center coin patch on backing plate and
drill rivet holes through coin patch and backing plate.
NOTE
The size and number of rivet holes
are determined by size of rivet to be
used.
In
most
cases
rivets
MS20470AD3 can by used.
(5)
Install and set rivets.
(6)
Spray or brush a thin coat of primer (item
113, Appendix C) on all surfaces of coin patch. Allow
primer to dry.
2-59
