FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 4. Metal Structure, Welding, and Brazing | 3. Precautionary Measures | 4-37. Load Factors for Repairs
AC 43.13-1B
9/8/98
4 inches and less than or equal to 35 percent of
the control surface span would have a maxi
mum permissible free play of 4 inches x 0.020
or 0.080 inches (total motion up and down)
measured at the trailing edge. Correct any free
play in excess of this amount.
(2) Care must also be exercised during
repair or rework to prevent stress concentration
points or areas that could increase the fatigue
susceptibility of the trim tab system. Advisory
Circular (AC) 23.629-1A, Means of Compli
ance with Section 23.629, “Flutter,” contains
additional information on this subject.
NOTE: If the pilot has experienced
flutter, or thinks he/she has, then a
complete inspection of the aircraft
flight control system and all related
components including rod ends,
bearings, hinges, and bellcranks must
be accomplished. Suspected parts
should be replaced.
4-37. LOAD FACTORS FOR REPAIRS.
In order to design an effective repair to a sheet
metal aircraft, the stresses that act on the
structure must be understood.
4-38. TRANSFER OF STRESSES WITH-
IN A STRUCTURE. An aircraft structure
must be designed in such a way that it will ac
cept all of the stresses imposed upon it by the
flight and ground loads without any permanent
deformation. Any repair made must accept the
stresses, carry them across the repair, and then
transfer them back into the original structure.
These stresses are considered as flowing
through the structure, so there must be a con
tinuous path for them, with no abrupt changes
in cross-sectional areas along the way. Abrupt
changes in cross-sectional areas of aircraft
structure that are subject to cycle load
ing/stresses will result in stress concentration
that may induce fatigue cracking and eventual
failure. A scratch or gouge in the surface of a
highly-stressed piece of metal will cause a
stress concentration at the point of damage.
a. Multirow Fastener Load Transfer.
When multiple rows of rivets are used to se
cure a lap joint, the transfer of stresses is not
equal in each row. The transfer of stress at
each row of rivets may be thought of as trans
ferring the maximum amount capable of being
transferred without experiencing rivet shear
failure.
a. Six types of major stresses are known
and should be considered when making re
pairs. These are tension, compression, bend
ing, torsion, shear, and bearing
b. The design of an aircraft repair is
complicated by the requirement that it be as
light as possible. If weight were not critical,
repairs could be made with a large margin of
safety. But in actual practice, repairs must be
strong enough to carry all of the loads with the
required factor of safety, but they must not
have too much extra strength. A joint that is
too weak cannot be tolerated, but neither can
one that is too strong because it can create
stress risers that may cause cracks in other lo
cations.
b. Use Of Stacked Doublers. A stacked
doubler is composed of two or more sheets of
material that are used in lieu of a single,
thicker sheet of material. Because the stress
transferred at each row of rivets is dependent
upon the maximum stress that can be trans
ferred by the rivets in that row, the thickness of
the sheet material at that row need only be
thick enough to transfer the stress applied.
Employing this principle can reduce the weight
of a repair joint.
4-39.4-49. [RESERVED.]
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