FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 4. Magnetic Particle Inspection | 5-50. Special Examination Techniques
9/8/98
AC 43.13-1B
(7) The intensity of the longitudinal
shots is kept just below the level at which
leakage fields develop across sharp changes of
section, such as radii under bolt heads, threads,
and other sharp angles in parts. This does not
apply when checking chrome-plated parts for
grinding cracks.
(a) For example, longitudinal mag-
netization of a round steel bar would be pro-
duced by placing the DC coil around the bar.
After application of the magnetic particles, ei-
ther during or subsequent to magnetization,
discontinuities perpendicular to the longitudi-
nal axis of the bar would be disclosed.
(b) When a yoke is used, the portion
of the part between the ends of the yoke com-
pletes the path of the magnetic lines of force.
This results in a magnetic field between the
points of contact.
c. Permanent Magnets and Electro-
magnetic Yoke. The stability of the magnetic
field generated by permanent magnets requires
some agitation of the oxide particles within the
field. The wet method is considered most sat-
isfactory. Use a well-agitated plastic squirt
bottle for the most effective application of the
magnetic particle suspension. When the di-
rection of possible cracks in a suspect area is
not known, or would not necessarily be normal
to the lines of force between the poles of the
magnet, reposition the magnet to the best ad-
vantage and recheck. Usually, two shots,
90 degrees apart are required. The part must
be demagnetized between each magnetization
when the field direction is changed unless the
next shot is at least 10 percent stronger than
the previous shot, if this is the case demagneti-
zation is not necessary.
5-49. DETERMINATION OF FIELD
STRENGTH. Factors such as part size,
shape, magnetic properties of the material, and
the method of magnetization will affect the
field strength induced within a part by a given
applied magnetizing force. The factors vary
considerably, making it difficult to establish
rules for magnetizing during examination.
Technique requirements are best determined
on actual parts having known defects.
a. A magnetization indicator, such as a
Quantitative Quality Indicator (QQI), should
be used to verify that adequate magnetic flux
strength is being used. It effectively indicates
the internally-induced field, the field direction,
and the quality of particle suspension during
magnetization.
b. The level of magnetization required
for detection of service-related defects in most
cases can be lower than that required for mate-
rial and manufacturing control. Contact the
manufacturer for correct specifications.
NOTE: If the examination must be
performed with less current than is
desired because of part size or equip-
ment limitations, the lower field
strength can be partially accommo-
dated by reducing the area of exami-
nation for each magnetization, or the
examination can be supplemented by
using electromagnetic yokes. Examine
only 4 inches on either side of a coil
instead of 6, or apply additional mag-
netization around the periphery of a
hollow cylinder when using an inter-
nal conductor.
5-50. SPECIAL EXAMINATION TECH-
NIQUES.
a. Magnetic Rubber. Magnetic rubber
formulations using finely divided magnetic
particles in a silicone rubber base are used for
the inspection of screw, bolt, or other bore
holes, which are not easily accessible. The
liquid silicone rubber mixture is poured into
holes in magnetic parts to be inspected.
Par 5-48
Page 5-25