Cessna 172 Skyhawk SERIES, Maintenance Manual

Page: 81 / 167

CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
FUSELAGE DAMAGE CLASSIFICATION
1. General
A. Damage to the fuselage can be divided into three major categories; negligible damage, repairable
damage, and major replacement damage. The categories are provided to assist in determining the
extent and criticalness of any damage.
2. Negligible Damage
A. Any smooth dents in the fuselage skin that are free from cracks, abrasions, and sharp corners, and
which are not stress wrinkles and do not interfere with any internal structure or mechanism, may
be considered as negligible damage. In areas of low stress intensity, cracks, deep scratches, or
deep, sharp dents - which after trimming or stop-drilling can be enclosed by a two-inch circle - can
be considered negligible if the damaged area is at least one diameter of the enclosing circle away
from all existing rivet lines and material edges. Stop drilling is considered a temporary repair and a
permanent repair must be made as soon as practical.
B. Mild corrosion appearing upon clad aluminum surfaces does not necessarily indicate incipient failure of
the base metal. However, corrosion of all types must be carefully considered, and approved remedial
action taken.
C. Small cans appear in the skin structure of all metal airplanes and should not necessarily be a cause
for concern. However. It is strongly recommended that wrinkles which appear to have originated from
other sources, or which do not follow the general appearance of the remainder of the skin panels, be
thoroughly investigated. Except in the landing gear bulkhead areas, wrinkles occurring over stringers
which disappear when the rivet pattern is removed, may be considered negligible. However, the
stringer rivet holes may not align perfectly with skin holes because of a permanent "set" in the stringer.
If this is apparent, replacement of the stringer will usually restore the original strength characteristics
of the area.
NOTE: Wrinkles occurring in the skin of the main landing gear bulkhead areas must not be
considered negligible. The skin panel must be opened suf
fi ciently to permit a thorough
examination of the lower portion of the landing gear bulkhead and its tie-in structure.
D. Wrinkles occurring in open areas which disappear when the rivets at the edge of the sheet are
removed, or a wrinkle which is hand removable, may often be repaired by a 1/2 inch x 1/2 inch x
0.050 inch 2024-T42 extruded angle or a heavy “J” section. The angle should be inserted fore and
aft across the center of the wrinkle and should extend to within 1/16 inch to 1/8 inch of the fuselage
bulkheads comprising the end of the bay. Rivet pattern should be similar to existing manufactured
seam at edge of sheet.
E. Negligible damage to stringers, formed skin fl anges, bulkhead channel and like parts is similar to
that for the wing skin. Refer to Chapter 57, Wing Damage Classi fi cation for a de fi nition of negligible
damage to these components.
3. Repairable Damage
A. If a skin is badly damaged, repair must be made by replacing an entire skin panel, from one structural
member to the next. Repair seams must be made to lie along structural members and each seam
must be made exactly the same in regard to rivet size, spacing and pattern as the manufactured
seams at the edges of the original sheet. If the manufactured seams are different, the stronger must
be copied. If the repair ends at a structural member where no seam is used, enough repair panel
must be used to allow an extra row of staggered rivets, with suf
fi cient edge margin to be installed.
B. Typical methods of repair for skins, bulkheads, stringers, and channels are illustrated in Chapter 51,
Typical Skin Repairs. Before repairs are attempted, all cracks or deep scratches must be stop-drilled
with a No. 30 (0.128 inch) drill and all sharp corners and ragged edges must be trimmed away and
deburred.
53-10-00
Page 1
© Cessna Aircraft Company
Jun 1/2005