FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 2. Visual Inspection | 5-18. Visual Inspection Procedures
AC 43.13-1B
9/8/98
b. Inspection Mirrors. An inspection
mirror is used to view an area that is not in the
normal line of sight. The mirror should be of
the appropriate size to easily view the compo-
nent, with the reflecting surface free of dirt,
cracks, worn coating, etc., and a swivel joint
tight enough to maintain its setting.
c. Simple Magnifiers. A single con-
verging lens, the simplest form of a micro-
scope, is often referred to as a simple magni-
fier. Magnification of a single lens is deter-
mined by the equation M = 10/f. In this equa-
tion, “M” is the magnification, “f” is the focal
length of the lens in inches, and “10” is a con-
stant that represents the average minimum
distance at which objects can be distinctly seen
by the unaided eye. Using the equation, a lens
with a focal length of 5 inches has a magnifi-
cation of 2, or is said to be a two-power lens.
5-17. BORESCOPES. These instruments
are long, tubular, precision optical instruments
with built-in illumination, designed to allow
remote visual inspection of internal surfaces or
otherwise inaccessible areas. The tube, which
can be rigid or flexible with a wide variety of
lengths and diameters, provides the necessary
optical connection between the viewing end
and an objective lens at the distant, or distal tip
of the borescope. Rigid and flexible bores-
copes are available in different designs for a
variety of standard applications and manufac-
turers also provide custom designs for spe-
cialized applications. Figure 5-1 shows three
typical designs of borescopes.
a. Borescopes Uses. Borescopes are used
in aircraft and engine maintenance programs to
reduce or eliminate the need for costly tear-
downs. Aircraft turbine engines have access
ports that are specifically designed for bores-
copes. Borescopes are also used extensively in
a variety of aviation maintenance programs to
determine the airworthiness of difficult-to-
reach components.
Borescopes
typically are used to inspect interiors of hy-
draulic cylinders and valves for pitting, scor-
ing, porosity, and tool marks; inspect for
cracked cylinders in aircraft reciprocating en-
gines; inspect turbojet engine turbine blades
and combustion cans; verify the proper place-
ment and fit of seals, bonds, gaskets, and sub-
assemblies in difficult to reach areas; and as-
sess Foreign Object Damage (FOD) in aircraft,
airframe, and powerplants. Borescopes may
also be used to locate and retrieve foreign ob-
jects in engines and airframes.
b. Optical Designs. Typical designs for
the optical connection between the borescope
viewing end and the distal tip are:
(1) A rigid tube with a series of relay
lenses;
(2) A flexible or rigid tube with a bun-
dle of optical fibers; and
(3) A flexible or rigid tube with wiring
that carries the image signal from a Charge
Couple Device (CCD) imaging sensor at the
distal tip.
These designs can have either fixed or adjust-
able focusing of the objective lens at the distal
tip. The distal tip may also have prisms and
mirrors that define the direction and field of
view. A fiber optic light guide with white
light is generally used in the illumination sys-
tem, but ultraviolet light can also be used to
inspect surfaces treated with liquid fluorescent
penetrant or to inspect for contaminants that
fluoresce. Some borescopes with long work-
ing lengths use light-emitting diodes at the
distal tip for illumination.
5-18. VISUAL INSPECTION PROCE-
DURES. Corrosion can be an extremely criti-
cal defect. Therefore, NDI personnel should
be familiar with the appearance of common
types of corrosion and have training and
Page 5-10
Par 5-16