inspecting internal structures for damage or signs of strain. These
members must be repaired or replaced if they are bent, fractured, or
wrinkled.
All rivets in the vicinity of such members must be
inspected for signs of failure.
They can be sheared without such a
condition being externally evident. Under such circumstances, rivets
at various points in the damaged area must be drilled out and
examined.
For this procedure the applicable TM for the particular
aircraft must be consulted.
The paragraphs that follow discuss these aspects of skin replacement
and the mechanics and techniques in measuring, cutting, and aligning
new metal sheets, drilling new rivet holes, and bucking rivets.
Measuring, Cutting, and Aligning.
In removing a panel, care must
be taken to avoid further skin damage because frequently the old skin
can be used as a template for the new skin.
The gage and alloy of
material to replace the panel is shown in the applicable manual for
the specific aircraft.
The size of the panel can be determined in
either of two ways.
The dimensions can be measured during the
inspection, or the old skin can be used as a template for the layout
of the new sheet of skin and the location of the holes. Because the
latter method is more accurate, it is the preferred one.
Whichever
method is used, the new sheet must be large enough to replace the
damaged area; and it can be cut with an allowance of 1 to 2 inches of
material outside the rivet holes.
If the old sheet is not too badly damaged, it must be flattened out
and can be used as a template.
The new sheet, cut 1 to 2 inches
larger than the old, is drilled near the center using the holes in
the old sheet as a guide. The two sheets are then fastened together
with sheet metal fasteners.
Sheet metal screws are not recommended
because they damage the edges of the rivet holes.
Drilling must
always start at the center and work to the outside of the sheet with
sheeting metal fasteners inserted at frequent intervals.
If the old sheet cannot be used as a template, the holes in the new
sheet must be drilled from the inside of the structure. The holes in
the reinforcing members are used as guides for the drilling.
Fasteners are inserted in the same manner as described before; this
technique is called back drilling. Before the new sheet is placed on
the framework to drill the holes, the reinforcing members must be in
proper alignment and flush at the points where they intersect.
Therefore, the new sheet must have the same contour as the old sheet.
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AL0992
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