Cessna 172 Maintenance Manual page 41

(b) Tipping of the fastener head may indicate its looseness or slippage. Rivet head periphery
rolled upward also indicates looseness.
A black or dark gray stain is found adjacent to or around the fastener head. Generally, it
(c)
takes the form of a dirt or oily streak aft of the loose rivet.
(d) Mark a red line across the fastener head and the adjacent material. Check the line at the
next inspection. Any loosening of the fastener will break the line as indicated in Figure 3.
9. Spacing Of Rivets
A. There are no specific rules which are applicable to every case or type of riveting. There are, however,
certain general rules which should be understood and followed. Edge distance of rivets should not
be less than two diameters of the rivet, measured from the edge of the sheet or plate to the center of
the rivet hole. Spacing between rivets, when in rows, depends upon several factors, principally the
thickness of the sheet, the diameter of the rivets, and the manner in which the sheet will be stressed.
This spacing is seldom less than four diameters of the rivet, measured between the centers of the rivet
holes. Rivets, spaced four diameters apart, are found in certain seams of semimonocoque fuselages,
webs or built up spars, and various plates or fittings. Where there are two rows of rivets, they are
usually staggered. The transverse pitch or distance between rows should be slightly less than the
pitch of the rivets, with 75 percent of the rivet pitch being the usual practice. An average spacing or
pitch of rivets in the cover or skin of most structures, except at highly stressed points, will be from 6
to 12 diameters of the rivet. The best practice in repair is to make pitch of rivets equal to those in the
original structure.
10. Threaded Fasteners Bolt Torques
The importance of correct application cannot be overemphasized. Refer to appropriate Maintenance
A.
Manual, Chapter 20, Torque Data - Maintenance Practices, for additional information covering torque
values. Under torque can result in unnecessary wear of nuts and bolts as well as parts they are
holding together. When insufficient pressures are applied, uneven loads will be transmitted throughout
assembly, which may result in excessive wear or premature failure due to fatigue. Over torque can
be equally damaging because of failure of a bolt or nut from overstressing threaded areas. There are
a few simple, but very important, procedures that should be followed to assure that correct torque is
applied:
(1) Calibrate torque wrench periodically to assure accuracy, and recheck frequently.
Be sure that bolt and nut threads are clean and dry unless otherwise specified.
(2)
(3) Run nut down to near contact with washer or bearing surface and check friction drag torque
required to turn nut.
(4) Add friction drag torque to desired torque recommended. Refer to appropriate Maintenance
Manual, Chapter 20, Torque Data - Maintenance Practices to obtain complete torque calculating
procedures. This is referred to as final torque which should register on indicator or setting for a
snap over-type wrench.
(5) Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs
during final torque, back off and re-torque.
When installing a castellated nut, start alignment with cotter pin hole at minimum recommended
(6)
torque plus friction drag torque, and do not exceed maximum torque plus friction drag. If hole
and nut castellation do not align, change washers or nut and try again. Exceeding maximum
recommended torque is not recommended unless specifically allowed or recommended for that
particular installation.
11. Rivets for Plastic or Composite Parts
A. Unlike rivets in metallic joints, blind rivets are often the rivet of choice for riveting non-metallic materials
because they may be installed without the hammering necessary to install solid rivets. If the tail end
of the rivet is adjacent to the non-metal side, install a washer over the shank to prevent the "hole
filling" action built into blind rivets from overloading the non-metal hole. The hole in the washer should
match the specified installation hole for the fastener. If the tail end of the rivet is installed through
metal substructure, the washer is not necessary.
CESSNA AIRCRAFT COMPANY
SINGLE ENGINE
STRUCTURAL REPAIR MANUAL
© Cessna Aircraft Company
51-40-00
Page 10
Jun 1/2005