However, the direct and general support (DS and GS) maintenance
manual applicable to the particular aircraft gives essential
information to accomplish direct or general support and depot
maintenance on the complete airframe, including the kind, size, and
number of rivets.
ORIGINAL CONTOUR
All repairs must be made so that the original contour is maintained
exactly. This is especially true with patches on the external skin.
Changes in external contour mean changes in air flow across the
external skin. These changes can cause vibrations that might lead to
other damage.
MAINTAIN MINIMUM WEIGHT
All repairs must be kept to
a minimum weight. Keep the patch size as
small as practicable and
limit the number of rivets to what is
necessary.
In many cases,
repairs disturb the structure's original
balance. By adding weight
in each repair, a control surface becomes
so unbalanced that the trim
and balance tabs will require adjusting.
SUMMARY
All repairs in aviation maintenance must maintain the original
strength of the repaired part or member.
For example, if the
fuselage skin is subject to compression, put the patch on the
outside; this ensures greater resistance to such loads.
The patch
cross-sectional area must be equal to, or greater than, the
original damaged area or section.
To reduce the danger of cracks
starting at the corners, use circular or oval patches.
If a
rectangular patch must be used, you should ensure that the curvature
radius at each corner is no smaller than 1/2 inch. Buckled or bent
members are replaced or reinforced by putting splices over the
affected areas.
Replacement or reinforcement material must be
similar to the original material. If substitutions must be made, be
sure that the material's gage is heavy enough to give equivalent
cross-sectional strength. Using a lighter gage, stronger material is
dangerous. One material may have more tensile strength but
less compressive strength than another.
Buckling and torsional
strengths of some sheet metal and tubular parts depend primarily upon
thickness rather than allowable compressive and shear strengths. So,
even though a thinner material has higher compressive and
shear strength, it can have less buckling and torsional strength.
Care must be used when forming heat-treated and cold-worked alloys.
When subjected to much bending, such alloys will crack.
Although
soft alloys are easily formed without danger of cracking, they
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