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 | 4. Metal Repair Procedures | 4-52. Aluminum Alloy Structures
9/27/01
AC 43.13-1B CHG 1
SECTION 4. METAL REPAIR PROCEDURES
4-50. GENERAL. The airframe of a fixed-
wing aircraft is generally considered to consist
of five principal units; the fuselage, wings,
stabilizers, flight control surfaces, and landing
gear.
a. Aircraft principal structural elements
(PSE) and joints are designed to carry loads by
distributing them as stresses. The elements
and joints as originally fabricated are strong
enough to resist these stresses, and must re
main so after any repairs. Long, thin elements
are called members. Some examples of mem
bers are the metal tubes that form engine
mount and fuselage trusses and frames, beams
used as wing spars, and longerons and string
ers of metal-skinned fuselages and wings.
Longerons and stringers are designed to carry
principally axial loads, but are sometimes re
quired to carry side loads and bending mo
ments, as when they frame cutouts in
metal-skinned structures. Truss members are
designed to carry axial (tension and compres
sion) loads applied to their ends only. Frame
members are designed to carry side loads and
bending moments in addition to axial loads.
Beam members are designed to carry side
loads and bending moments that are usually
large compared to their axial loads. Beams
that must resist large axial loads, particularly
compression loads, in combination with side
loads and bending moments are called
beam-columns. Other structural elements such
as metal skins, plates, shells, wing ribs, bulk
heads, ring frames, intercostal members, gus
sets, and other reinforcements, and fittings are
designed to resist complex stresses, sometimes
in three dimensions.
b. Any repair made on an aircraft struc
ture must allow all of the stresses to enter,
sustain these stresses, and then allow them to
return into the structure. The repair must be
equal to the original structure, but not stronger
or stiffer, which will cause stress concentra
tions or alter the resonant frequency of the
structure.
c. All-metal aircraft are made of very
thin sheet metal, and it is possible to restore
the strength of the skin without restoring its ri
gidity. All repairs should be made using the
same type and thickness of material that was
used in the original structure. If the original
skin had corrugations or flanges for rigidity,
these must be preserved and strengthened. If a
flange or corrugation is dented or cracked, the
material loses much of its rigidity; and it must
be repaired in such a way that will restore its
rigidity, stiffness, and strength.
4-51. RIVETED (OR BOLTED) STEEL
TRUSS-TYPE STRUCTURES. Repairs to
riveted structures may be made employing the
general principles outlined in the following
paragraphs on aluminum alloy structures. Re
pair methods may also be found in text books
on metal structures. Methods for repair of the
major structural members must be specifically
approved by the Federal Aviation Administra
tion (FAA).
4-52. ALUMINUM ALLOY STRUC
TURES. Extensive repairs to damaged
stressed skin on monocoque-types of alumi
num alloy structures must be made in accor
dance with FAA-approved manufacturer’s in
structions or other FAA-approved source.
a. Rivet Holes. Rivet holes are slightly
larger than the diameter of the rivet. When
driven, solid rivets expand to fill the hole. The
strength of a riveted joint is based upon the
expanded diameter of the rivet. Therefore, it is
important that the proper drill size be used for
each rivet diameter.
Par 4-50
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