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 | 5-97. Leak Testing
AC 43.13-1B
9/8/98
operated near a surface edge. Edge effects on
vibration paths give a test reading that may be
misinterpreted. To avoid edge effects, the test
probe should be moved so that the inspection
path follows the surface edge, giving a con-
stant edge for the test probe to inspect. Edge
effects are more pronounced in thicker mate-
rial. To interpret meter readings correctly, the
operator should determine whether there are
variations in the thickness of the material.
c. Probe Sending Signal. With the ex-
ception of the Sondicator models, the test
probes of the testers emit a sending signal that
radiates in a full circle. The sending signal of
the Sondicator probe travels from one trans-
ducer tip to the other. For this reason, the test
probe should be held so that the transducer tips
are at right angles to the inspection path.
When inspecting honeycomb panels with a
Sondicator model, the transducer tips should
be moved consistently in the direction of the
ribbon of the honeycomb or at right angles to
the ribbon so that a constant subsurface is pre-
sented.
5-96. THICKNESS MEASUREMENTS.
Ultrasonic inspection methods can be used for
measurement of material thickness in aircraft
parts and structures.
a. Applications. Ultrasonic thickness
measurements are used for many applications,
such as: checking part thickness when access
to the back side is not available; checking large
panels in interior areas where a conventional
micrometer cannot reach; and in maintenance
inspections for checking thickness loss due to
wear and/or corrosion.
b. Pulse-Echo Method. The most com-
monly used ultrasonic thickness measurement
method. The ultrasonic instrument measures
time between the initial front and back surface
signals or subsequent multiple back reflection
signals. Since the velocity for a given material
is a constant, the time between these signals is
directly proportional to the thickness. Cali-
bration procedures are used to obtain direct
readout of test part thickness.
c. Thickness Measurement Instrument
Types. Pulse-echo instruments designed ex-
clusively for thickness measurements are gen-
erally used in lieu of conventional pulse-echo
instruments; however, some conventional
pulse-echo instruments also have direct thick-
ness measurement capabilities. Conventional
pulse-echo instruments without direct thick-
ness measuring capabilities can also be used
for measuring thickness by using special pro-
cedures.
d. Thickness Measurement Ranges.
Dependent upon the instrument used and the
material under test, material thickness
from 0.005 inches to 20 inches (or more) can
be measured with pulse-echo instruments de-
signed specifically for thickness measuring.
5-97. LEAK TESTING. The flow of a
pressurized gas through a leak produces sound
of both sonic and ultrasonic frequencies. If the
gas leak is large, the sonic frequency sound it
produces can probably be detected with the ear
or with such instruments as stethoscopes or
microphones; however, the ear and these in-
struments have limited ability to detect and lo-
cate small leaks. Ultrasonic leak detectors are
frequently used to detect leaks that cannot be
detected by the above methods, because they
are very sensitive to ultrasonic energy and, un-
der most conditions, background noise at other
frequencies does not affect them.
a. Standard Method. A standard method
of testing for leaks using ultrasonics is pro-
vided in ASTM E 1002. The method covers
procedures for calibration, location, and esti-
mated measurements of leakage by the ultra-
sonic technique (sometimes called ultrasonic
translation).
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