FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 7. Ultrasonic Inspection | 5-94. Inspection of Bonded Structures
9/8/98
AC 43.13-1B
(3) Underlying structures contain dif-
ferences in; core material, cell size, thickness,
and height, back skin material and thickness,
doublers (material and thickness), closure
member attachments, foam adhesive, steps in
skins, internal ribs, and laminates (number of
layers, layer thickness, and layer material).
(4) The top only or top and bottom skin
of a bonded structure may be accessible.
b. Application of Ultrasonic Inspection.
Application to bonded structures must be ex-
amined in detail because of the many inspec-
tion methods and structural configurations.
The advantages and limitations of each in-
spection method should be considered, and
reference standards (representative of the
structure to be inspected) should be ultrasoni-
cally inspected to verify proposed techniques.
containing: closure members, core splices,
stepped skins, and internal ribs similar to the
test part if bonded areas over or surrounding
these details are to be inspected.
(2) The reference standard must contain
bonds of good quality except for controlled ar-
eas of unbond fabricated as explained below.
(3) The reference standard must be
bonded using the adhesive and cure cycle pre-
scribed for the test part.
e. Types of Defects. Defects can be sepa-
rated into five general types to represent the
various areas of bonded sandwich and laminate
structures as follows:
(1) Type I. Unbonds or voids in an
outer skin-to-adhesive interface.
c. Internal Configuration. Complete in-
formation on the internal configuration of the
bonded test part must be obtained by the in-
spector. Drawings should be reviewed, and
when necessary, radiographs of the test part
should be taken. Knowledge of details such as
the location and boundaries of doublers, ribs,
etc., is required for valid interpretation of ul-
trasonic inspection results. The boundaries of
internal details should be marked on the test
part using a grease pen or other easily remov-
able marking.
d. Reference Standards. Standards can
be a duplicate of the test part except for the
controlled areas of unbond. As an option,
simple test specimens, which represent the
varied areas of the test part and contain con-
trolled areas of unbond, can be used. Refer-
ence standards must meet the following re-
quirements.
(1) The reference standard must be
similar to the test part regarding material, ge-
ometry, and thickness. This includes
(2) Type II. Unbonds or voids at the
adhesive-to-core interface.
(3) Type III. Voids between layers of a
laminate.
(4) Type IV. Voids in foam adhesive or
unbonds between the adhesive and a closure
member at core-to-closure member joints.
(5) Type V. Water in the core.
f. Fabrication of NDI Reference Stan-
dards. Every ultrasonic test unit should have
sample materials that contain unbonds equal to
the sizes of the minimum rejectable unbonds
for the test parts. Information on minimum
rejectable unbond sizes for test parts should be
obtained from the OEM’s manuals, FAA re-
quirements, or the cognizant FAA Aircraft
Certification Office (ACO) engineer. One or
more of the following techniques can be used
in fabricating reference defects; however, since
bonding materials vary, some of the methods
may not work with certain materials.
Par 5-94
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