FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 1. General | 5-5. Flaws
9/8/98
AC 43.13-1B
(4) Porosity is holes in a material’s sur-
face or scattered throughout the material,
caused by gases being liberated and trapped as
the material solidifies.
(5) Inclusions are impurities, such as
slag, oxides, sulfides, etc., that occur in ingots
and castings. Inclusions are commonly caused
by incomplete refining of the metal ore or the
incomplete mixing of deoxidizing materials
added to the molten metal in the furnace.
(4) Cooling cracks can occur in casting
due to stresses resulting from cooling, and are
often associated with changes in cross sections
of the part. Cooling cracks can also occur
when alloy and tool steel bars are rolled and
subsequently cooled. Also, stresses can occur
from uneven cooling which can be severe
enough to crack the bars. Such cracks are gen-
erally longitudinal, but not necessarily straight.
They can be quite long, and usually vary in
depth along their length.
(6) Shrinkage cracks can occur in cast-
ings due to stresses caused by the metal con-
tracting as it cools and solidifies.
c. Primary Processing Flaws. Flaws
which occur while working the metal down by
hot or cold deformation into useful shapes
such as bars, rods, wires, and forged shapes are
primary processing flaws. Casting and weld-
ing are also considered primary processes al-
though they involve molten metal, since they
result in a semi-finished product. The follow-
ing are brief descriptions of some primary
processing flaws:
(1) Seams are surface flaws, generally
long, straight, and parallel to the longitudinal
axis of the material, which can originate from
ingot blowholes and cracks, or be introduced
by drawing or rolling processes.
(2) Laminations are formed in rolled
plate, sheet, or strip when blowholes or inter-
nal fissures are not welded tight during the
rolling process and are enlarged and flattened
into areas of horizontal discontinuities.
(3) Cupping is a series of internal metal
ruptures created when the interior metal does
not flow as rapidly as the surface metal during
drawing or extruding processes. Segregation
in the center of a bar usually contributes to the
occurrence.
(5) Flakes are internal ruptures that can
occur in metal as a result of cooling too rap-
idly. Flaking generally occurs deep in a heavy
section of metal. Certain alloys are more sus-
ceptible to flaking than others.
(6) Forging laps are the result of metal
being folded over and forced into the surface,
but not welded to form a single piece. They
can be caused by faulty dies, oversized dies,
oversized blanks, or improper handling of the
metal in the die. They can occur on any area
of the forging.
(7) Forging bursts are internal or exter-
nal ruptures that occur when forging opera-
tions are started before the material to be
forged reaches the proper temperature
throughout. Hotter sections of the forging
blank tend to flow around the colder sections
causing internal bursts or cracks on the sur-
face. Too rapid or too severe a reduction in a
section can also cause forging bursts or cracks.
(8) A hot tear is a pulling apart of the
metal that can occur in castings when the metal
contracts as it solidifies.
(9) A cold shut is a failure of metal to
fuse. It can occur in castings when part of the
metal being poured into the mold cools and
does not fuse with the rest of the metal into a
solid piece.
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