Cessna 172 Skyhawk SERIES, Maintenance Manual

Page: 83 / 167

CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
CABIN BULKHEAD REPAIR
1. General
A. Bulkheads are comprised of formed "C" channel sections. The principal material of construction
is 2024-0 Alclad aluminum alloy which, after forming, is heat-treated to a 2024-T42 condition. All
bulkheads in the fuselage are of the formed sheet metal or the reinforced formed sheet metal type.
2. Repair of Webs or Flanges
A. The following procedures are for the repair of cracked bulkhead webs or fl anges.
(1) Acceptable methods of repairing various types of cracks occurring in service are shown in
Figures 801 and 802.
(2) Stop-drill No. 30 (0.128 inch) minimum holes at extreme ends of cracks to prevent further
cracking.
(3) Reinforcements should be added to carry stresses across damaged portion and stiffen the joints.
NOTE: The condition causing such cracks to develop at a particular point may be stress
concentration at that point, in conjunction with repetition of stress (such as produced
by vibration of the structure). The stress concentration may be due to defects such
as nicks, scratches, tool marks, and initial stresses or cracks from forming or heat-
treating operations. An increase in sheet thickness alone is usually bene
fi cial but
does not necessarily remedy the condition leading to the cracking. Patch-type repairs
are generally employed and are usually satisfactory in restoring the original material
strength characteristics.
3. Repair of Channels
A. The following procedures are for the repair of severely bent, kinked, or torn channels.
(1)
If practical, severely bent, kinked, or torn portions of bulkheads should be removed and
replacement sections installed and joined at the original splice joint.
(2) If the procedure outlined in the preceding step is not justi fi ed, cutting away the damaged portion
and inserting a trimmed portion of the original section, adequately reinforced by splice plates or
doublers, will prove satisfactory. This is knownas an insertion-type patch.
4. Landing Gear Bulkheads
A. Landing gear bulkheads are highly stressed members, irregularly formed to provide clearance for
control cables, fuel and brake lines. Patch type repairs on these bulkheads are, for the most part,
impractical. Minor damage, consisting of small nicks or scratches, may be repaired by dressing out
the damaged area, or by replacement of fasteners. Any other damage must be repaired by replacing
the landing gear support assembly as an aligned unit.
5. Repair After Hard Landing
A. Buckled skin or fl oor boards, and loose or sheared rivets in the area of the main gear support are
indications of damage to structure from an extremely hard landing. When such evidence is present,
the entire support structure must be examined and all support forgings must be checked for cracks.
(1)
Use fl uorescent dye penetrant and magni fi cation to examine for cracks.
B. Bulkheads in the damaged area must be checked for alignment. Deformation of bulkhead webs must
be checked using a straightedge.
C. Damaged support structure, buckled fl oorboards and skins, and damaged or questionable forgings
must be replaced.
53-20-00
Page 801
© Cessna Aircraft Company
Jun 1/2005