FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 9. Acoustic Emission | 5-112. General
9/8/98
SECTION 8. TAP TESTING
AC 43.13-1B
5-105. GENERAL. Tap testing is widely
used for a quick evaluation of any accessible
aircraft surface to detect the presence of de-
lamination or debonding.
a. The tap testing procedure consists of
lightly tapping the surface of the part with a
coin, light special hammer with a maximum of
2 ounces (see figure 5-22), or any other suit-
able object. The acoustic response is com-
pared with that of a known good area.
b. A “flat” or “dead” response is consid-
ered unacceptable. The acoustic response of a
good part can vary dramatically with
changes in geometry, in which case a standard
of some sort is required. The entire area of
interest must be tapped. The surface should be
dry and free of oil, grease, and dirt. Tap test-
ing is limited to finding relatively shallow de-
fects in skins with a thickness less than
.080 inch. In a honeycomb structure, for ex-
ample, the far side bondline cannot be evalu-
ated, requiring two-side access for a complete
inspection. This method is portable, but no re-
cords are produced. The accuracy of this test
depends on the inspector’s subjective inter-
pretation of the test response; therefore, only
qualified personnel should perform this test.
FIGURE 5-22. Sample of special tap hammer.
5-106.5-111. [RESERVED.]
SECTION 9. ACOUSTIC-EMISSION
5-112. GENERAL. Acoustic-Emission is an
NDI technique that involves the placing of
acoustical-emission sensors at various loca-
tions on the aircraft structure and then applying
a load or stress. The level of stress applied
need not reach general yielding, nor does the
stress generally need to be of a specific type.
Bending stress can be applied to beamed
structures, torsional stress can be applied to
rotary shafts, thermal stresses can be applied
with heat lamps or blankets, and pres-
sure-induced stress can be applied to pres-
sure-containment systems such as the aircraft
fuselage. The materials emit sound and stress
waves that take the form of ultrasonic pulses
that can be picked up by sensors. Cracks and
areas of corrosion in the stressed airframe
structure emit sound waves (different frequen-
cies for different size defects) which are regis-
tered by the sensors. These acoustic-emission
bursts can be used both to locate flaws and to
evaluate their rate of growth as a function of
applied stress. Acoustic-emission testing has
an advantage over other NDI methods in that it
can detect and locate all of the activated flaws
in a structure in one test. Acoustic-emission
testing does not now provide the capability to
size flaws, but it can greatly reduce the area
required to be scanned by other NDI methods.
Par 5-105
Page 5-53