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 | 5-6. Selecting the Ndi Method
9/8/98
AC 43.13-1B
e. In-Service Flaws. These flaws are
formed after all fabrication has been completed
and the aircraft, engine, or related component
has gone into service. These flaws are attrib-
utable to aging effects caused by either time,
flight cycles, service operating conditions, or
combinations of these effects. The following
are brief descriptions of some in-service flaws.
(1) Stress corrosion cracks can develop
on the surface of parts that are under tension
stress in service and are also exposed to a cor-
rosive environment, such as the inside of wing
skins, sump areas, and areas between two
metal parts of faying surfaces.
(2) Overstress cracks can occur when a
part is stressed beyond the level for which it
was designed. Such overstressing can occur as
the result of a hard landing, turbulence, acci-
dent, or related damage due to some unusual or
emergency condition not anticipated by the de-
signer, or because of the failure of some re-
lated structural member.
(3) Fatigue cracks can occur in parts
that have been subjected to repeated or
changing loads while in service, such as riv-
eted lap joints in aircraft fuselages. The crack
usually starts at a highly-stressed area and
propagates through the section until failure oc-
curs. A fatigue crack will start more readily
where the design or surface condition provides
a point of stress concentration. Common
stress concentration points are: fillets; sharp
radii; or poor surface finish, seams, or grinding
cracks.
(4) Unbonds, or disbonds, are flaws
where adhesive attaches to only one surface in
an adhesive-bonded assembly. They can be the
result of crushed, broken, or corroded cores in
adhesive-bonded structures. Areas of unbonds
have no strength and place additional stress on
the surrounding areas making failure more
likely.
(5) Delamination is the term used to de-
fine the separation of composite material lay-
ers within a monolithic structure. Ultrasonic is
the primary method used for the detection of
delamination in composite structures.
5-6. SELECTING THE NDI METHOD.
The NDI method and procedure to be used for
any specific part or component will generally
be specified in the aircraft or component
manufacturer’s maintenance or overhaul
manuals, SSID’s, SB’s, or in AD’s.
NOTE: Some AD’s refer to SB’s
which may, in turn, refer to manufac-
turer’s overhaul or maintenance
manuals.
a. Appropriate Method. The appropriate
NDI method may consist of several separate
inspections. An initial inspection may indicate
the presence of a possible flaw, but other in-
spections may be required to confirm the
original indication. Making the correct NDI
method selection requires an understanding of
the basic principles, limitations, and advan-
tages and disadvantages of the available NDI
methods and an understanding of their com-
parative effectiveness and cost.
b. Other Factors. Other factors affecting
the inspection are:
(1) The critical nature of the
component;
(2) The material, size, shape, and
weight of the part;
(3) The type of defect sought;
(4) Maximum acceptable defect limits
in size and distribution;
(5) Possible locations and orientations
of defects;
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