Cessna 172 Skyhawk SERIES, Maintenance Manual

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CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
DAMAGE INVESTIGATION AND CLASSIFICATION
1. General
A. For the purposes of this manual, damage is considered to be a deviation from the original
con
fi guration of a structural part that compromises its structural integrity by signi fi cantly reducing its
strength, signi fi cantly decreasing its resistance to fatigue, signi fi cantly increasing its susceptibility to
corrosion, signi fi cantly altering its fl utter characteristics, or adversely affecting the fl ight characteristics
of the airplane. This can include - but is not limited to - scratches, dents, dings, gouges, cracks,
drill starts, double drilled holes, plastic deformation, reduction in cross-sectional areas, changes in
component center-of-gravity, missing or inadequate fasteners, corrosion, dissimilar metal contact,
work hardening, temper change due to excessive heat, and so forth.
B. Use good judgment in determining the type of signi fi cant change to fl at stock structural material. The
terms, dent, crease, abrasion, gouge, nick, scratch, crack and corrosion, referred to elsewhere in the
manual, are de
fi ned below as a guide for this determination, particularly with respect to the external
skin of the airplane:
(1)
Dent - A dent is normally a damaged area which is depressed with respect to its normal contour.
There is no cross sectional area change in the material. Area boundaries are smooth. Its form
is generally the result of contact with a relatively smoothly contoured object.
NOTE: A dent-like form of damage to skin may be the result of the peening action of a
smoothly contoured object contacting it. If the inner surface of skin shows no contour
change, consider that such damage results in a local cross sectional area change.
(2) Crease - A damaged area which is depressed or folded back upon itself in such a manner that its
boundaries are sharp or well de
fi ned lines or ridges. Consider it to be the equivalent of a crack.
(3) Abrasion - An abrasion is a damaged area of any size which results in a cross sectional area
change due to scuf
fi ng, rubbing, scraping or other surface erosion. It is usually rough and
irregular.
(4) Gouge - A gouge is a damaged area of any size, which results in a cross sectional area change.
It is usually caused by contact with a relatively sharp object which produces a continuous, sharp
or smooth channel-like groove in the material.
(5) Nick - A nick is a local gouge with sharp edges. Consider a series of nicks, in a line pattern to
be the equivalent of a gouge.
(6) Scratch - A scratch is a line of damage of any depth in the material and results in a cross sectional
area change. It is usually caused by contact with a very sharp object.
(7) Crack - A crack is a partial fracture or complete break in the material with the most signi fi cant
cross sectional area change. In appearance, it is usually an irregular line and is normally the
result of fatigue failure.
(8) Corrosion - Corrosion, due to a complex electrochemical action, is a damaged area of any size
and depth which results in a cross sectional area change. Depth of such pitting damage must
be determined by a cleanup operation. Damage of this type may occur on surfaces of structural
elements. Refer to Corrosion and Corrosion Control, Section 51-11-00.
C. Use good sense and proper visual measurement in the determination of signi fi cant cross sectional
area changes of both depth and length of any type (or combinations) of damage mentioned above.
2. Damage Investigation
A. After a thorough cleaning of the damaged area, all structural parts should be carefully examined
to determine the extent of damage. Frequently, the force causing the initial damage is transmitted
from one member to the next, causing strains and distortions. Abnormal stresses incurred by shock
or impact forces on a rib, bulkhead, or similar structure, may be transmitted to the extremity of the
structural member, resulting in secondary damage, such as sheared or stretched rivets, elongated bolt
holes, or canned skins or bulkheads. Points of attachment should be examined carefully for distortion
and security of fastenings in the primary and secondary damaged areas at locations beyond the local
51-10-00
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© Cessna Aircraft Company
Jun 1/2005