FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 3. Eddy Current Inspection | 5-27. Field Application of Eddy Current Inspection
9/8/98
SECTION 3. EDDY CURRENT INSPECTION
AC 43.13-1B
5-25. EDDY CURRENT INSPECTION.
Eddy current is used to detect surface cracks,
pits, subsurface cracks, corrosion on inner sur-
faces, and to determine alloy and heat-treat
condition.
a. Eddy Current Instruments. A wide
variety of eddy current test instruments are
available. The eddy current test instrument
performs three basic functions: generating, re-
ceiving, and displaying. The generating por-
tion of the unit provides an alternating current
to the test coil. The receiving section proc-
esses the signal from the test coil to the re-
quired form and amplitude for display. In-
strument outputs or displays consist of a vari-
ety of visual, audible, storage, or transfer tech-
niques utilizing meters, video displays, chart
recorders, alarms, magnetic tape, computers,
and electrical or electronic relays.
FIGURE 5-3. Generating an eddy current.
b. Principles of Operations. Eddy cur-
rents are induced in a test article when an al-
ternating current is applied to a test coil
(probe). The alternating current in the coil in-
duces an alternating magnetic field in the arti-
cle which causes eddy currents to flow in the
article. (See figure 5-3.)
(1) Flaws in or thickness changes of the
test-piece influence the flow of eddy currents
and change the impedance of the coil accord-
ingly. (See figure 5-4.) Instruments display
the impedance changes either by impedance
plane plots or by needle deflection.
(2) Figure 5-5 shows typical impedance
plane display and meter display instrument re-
sponses for aluminum surface cracks, subsur-
face cracks, and thickness.
FIGURE 5-4. Detecting an eddy current.
5-26. EDDY CURRENT COILS AND
PROBES. A wide variety of eddy current
coils and probes is available. Coils and probes
are not always interchangeable between vari-
ous types of instruments and, for best results,
should be matched to a specific instrument and
frequency range. Special probe holders can be
fabricated to facilitate eddy current inspection
of contoured or shaped parts including part
edges.
5-27. FIELD APPLICATION OF EDDY
CURRENT INSPECTION. Eddy current
techniques are particularly well-suited for de-
tection of service-induced cracks in the field.
Par 5-25
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