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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 5-16PDF Image | AFS-640
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