marks; inspect for cracked cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans; verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult-to-reach areas of aircraft and aeronautical equipment; and assess Foreign Object Damage (FOD) in aircraft, airframe, and powerplants. Borescopes may also be used to locate and retrieve foreign objects in engines and airframes.
Optical Designs. Typical designs for the optical connection between the borescope viewing end and the distal tip are as follows: rigid tube with a series of relay lenses, flexible or rigid tube with a bundle of optical fibers, and flexible or rigid tube with wiring that carries the image signal from a CCD imaging sensor at the distal tip. These designs can have either fixed or adjustable focusing of the objective lens at the distal tip. The distal tip also has prisms and mirrors that define the direction and field of view (Figure 4-9). A fiber-optic light guide with white light is generally used in the illumination system, but ultraviolet light can also be used to inspect surfaces treated with liquid fluorescent penetrant or to inspect for contaminants that fluoresce. Some videoscopes with long working lengths use lightemitting diodes at the distal tip for illumination.
55° Bottoming 55* 55° Right angle Circumference 55° p Forward oblique Retrospective FIGURE 4-9. TYPICAL DIRECTIONS AND FIELD OF VIEW OF RIGID BORESCOPES (Courtesy of ASM International) Borescope Selection. Flexible and rigid borescopes are available in a wide variety of standard and customized designs, and several factors can influence the selection of a scope for Chap 4 Par 404 Page 129
particular application. These factors include focusing and resolution, illumination, magnification and field of view, working length, direction of view, and environment.
(1) Focusing and Resolution. In general, the optical quality of a rigid borescope improves as the size of the lens increases; consequently, a borescope with the largest possible diameter should be used. If portions of long objects are at different planes, the borescope should have sufficient focus adjustment to achieve an adequate depth of field. If the borescope has a fixed focal length, the object will be in focus only at a specific lens-to-object distance. To allow the observation of surface detail at a desired size, the optical system of a borescope should also provide adequate resolution and image contrast. If resolution is adequate, but contrast is lacking, detail cannot be observed. For fiberscopes, the resolution is dependent on the accuracy of alignment and the diameter of the fibers in the image bundle. Smaller diameter fibers provide more resolution and edge contrast when combined with good geometrical alignment of the fi-
(2) Illumination. The amount of illumination depends on the diameter of the light guide bundle; therefore, it is desirable to use the largest diameter possible. The required intensity of the light source is determined by the reflectivity of the surface, the area of surface to be illuminated, and the transmission losses over the length of the scope. Rigid borescopes with a lamp at the distal end provide the greatest amount of illumination over the widest area at working lengths greater than 20 feet; however, the heat generated by the light source can damage some materials. Fiber-optic illumination is usually brighter than a lamp at the distal end with scope working lengths less than 20 feet and is suitable for heat-sensitive applications when filters are used to remove infrared wavelengths.
(3) Magnification and Field of View. Magnification and field of view are interrelated, and as magnification is increased, the field of view is reduced. The precise relationship between magnification and field of view is generally specified by the product manufacturer. The degree of magnification in a particular application is determined by the field of view and the distance from the objective lens to the object. The magnification increases when either the field of view or the lens-toobject distance decreases.
(4) Working Length. The working length can sometimes dictate the use of a particular type of borescope. For example, a rigid borescope with a long working length may be limited by the need for additional supports. In general, videoscopes allow a longer working length than fiberscopes.
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8/14/97 AU 43-204 Direction of View. The selection of a viewing direction is influenced by the location of the access port in relation to the object to be observed. Flexible fiberscopes or videoscopes, because of their articulating tip, are often adequate with either a side or forward viewing tip. Circumferential or panoramic heads are designed for the inspection of tubing or other cylindrical parts. A centrally located mirror permits right angle viewing of an area just scanned by the panoramic The forward viewing head permits the inspection of the area directly ahead of the viewing head and is commonly used when examining facing walls or the bottoms of blind holes and cavities. Forward-oblique heads bend the viewing direction at an angle to the borescope axis, permitting the inspection of corners at the end of a bored hole. Retrospective viewing heads bend the cone of view at a retrospective angle to the borescope axis, providing a view of the area just passed by the advancing borescope and are especially suited for inspecting the inside corners of valve body and actuating cylinder castings.
Environment. Flexible and rigid borescopes can be manufactured to withstand a variety of environments. Although most borescopes can operate at temperatures from -30 to 150 degrees F, specially designed borescopes can be used at temperatures to 3500 degrees F. Borescopes can also be manufactured for use in liquid media. Special borescopes are required for use at pressures above ambient and in atmospheres exposed to radiation.
Radiation can cause lenses and fiber-optic bundles to turn brown.
when a borescope is used in atmospheres exposed to radiation, quartz fiberscopes are generally used. Scopes used in a gaseous environment should be made explosion-proof to minimize the potential of an accidental explosion.
Rigid Borescopes. Rigid borescopes are generally used in aircraft maintenance applications where there is a straight line access path to the area to be inspected. Some typical uses for rigid borescopes in aircraft maintenance are inspection of landgear actuators and struts for corrosion; cracks and scratches on bore walls; inspection for corrosion and cracks of wing stringers, wing attachment fittings, spars, and other aircraft structural members; inspection of engine cylinder sleeves, pistons, and valves for damage; and inspection of aircraft turbines for cracks, corrosion, damage by foreign objects, and erosion.
Rigid borescope sizes range in lengths from 2 inches to over 100 feet and in diameters from 0.035 to 2.75 inches. Magnification is usually 3 to 4 power, but powers up to 50 are available. They are available with fixed and adjustable focus eyepiece. The illumination system is either an incandescent lamp located at the distal end or a light-guide bundle made from optical fibers that conducts light from an external source. The fiber-optics, lightguide bundle has no electrical contacts at the distal end and produces cold light to minimize explosion hazards. Rigid borescopes are available with a variety of viewing heads for se- Chap 4 Par 404 Page 131
AC 43 - 204 8/14/97 lection according to the application. They generally have a 55- degree field of view although fields of view can range from 10 to 90 degrees. The distal tips are generally not interchangeable, bility to rotate the optical shaft from 120 to 370 degrees for scanning purposes, and others have movable prisms at the tip for longitudinal scanning.
(1) Basic Design. The rigid borescope typically has a series of achromatic relay lenses in the optical tube.
lenses preserve the resolution of the image as it travels from the objective lens to the eyepiece. The tube diameter of these borescopes ranges from 0.035 to 2.75 inches. The illumination system can be either a distal lamp or a light-guide bundle, and the various features may include rotating optical shait, various viewing heads, and adjustable focusing of the objective lens.
(2) Micro-borescopes. Micro-borescopes are smalldiameter borescopes having an integral light-guide bundle and a one-piece rod lens for maximum resolution with a minimum diameter. Some typical uses for micro-borescopes in aircraft maintenance are inspection for damage within fuel nozzles, electronic components, instruments, hydraulic components, castings, and other miniature equipment. The lengths of micro borescopes range from 2 to 32 inches, and the diameters range from 0.020 to 0.125 inch. They are available with forward, forward oblique, and right angle views; support stands with micrometer adjustable fixturing; photo and video adapters; up to 30-power magnification;
and fixed or ocular focus. Figure 4-10 shows a typical microborescope.
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FIGURE 4-10. TYPICAL MICRO-BORESCOPE (Courtesy of Industry Technology, Inc.)
(3) Extendible Borescopes. Extendible borescopes allow the user to vary the length of the borescope tube by inserting extension tubes. Some typical uses for rigid extendible borescopes in aircraft maintenance are inspection for damage within turbine engine combustion cases and inspection for cracks in aircraft rear pressure bulkheads. Extendible borescopes are available with either a fiber-optic light guide or an incandescent lamp at the distal end. Extendible borescopes with an integral lamp have a maximum length of about 100 feet.
borescopes with a light-guide bundle are available with smaller tube diameters, but have shorter maximum lengths (approximately 30 feet).
work of several different borescopes. Therefore, when feasible for the inspection tasks involved, they can have the advantage of economy, adaptability, and less bulk and weight for easy transportation and use over the purchase and use of a different borescope for each inspection task. Interchangeable viewing heads are available for extendible borescopes. Figure 4-11 shows a typical extendible borescope.
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FIGURE 4-11. TYPICAL EXTENDIBLE BORESCOPE (Courtesy of Titan Tool Supply Company, Inc.)
Endoscopes. The endoscope is much like a borescope with a high-intensity light source, except that it has a superior optical system which remains constantly in focus from about 4 mm
(0.16 inch) to infinity. Actually, when the tip is about 4 mm rom the suriace, a magniiication oi about lux is achleved.
doscopes are available in diameters down to 1.7 mm (0.07 inch) and in lengths from 100 to 150 mm (4 to 6 inches).
(5) Mirror Adapters. Mirror adapters can be used to convert a direct-viewing borescope into a side-viewing borescope.
A mirror adapter is designed to fit over the tip of the borescope and reflect an image from the side of the borescope. However, there is some reduction in resolution and image contrast; therefore, not all applications are suitable for this device. A side, forward oblique, or retrospective viewing head provides better resolution and a higher degree of image contrast than a directviewing borescope with a mirror adapter. A mirror adapter also produces an inverse image and can produce unwanted reflections from the shaft.
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8/14/97 10 43-204
(6) Scanning. Some rigid borescopes have the ability to scan longitudinally along the axis of the shaft. A movable prism with a control at the handle accomplishes this scanning. Typically, the prism can shift the direction of view through an arc of 120 degrees. Figure 4-12 shows a typical scanning borescope.
lupus toscope Prem contro Abortiont connector FIGURE 4-12. TYPICAL SCANNING BORESCOPE (Courtesy of Richard Wolf Medical Instruments Corporation) Zooming Borescopes. This instrument does not entirely fit the definition of a rigid borescope; however, it is used for inspection of bores and cavities. The zooming borescope (see Figure 4-13) can be used in aircraft maintenance for checking the whole interior surface of tubing and bores for surface -inish, cracks, dirt, weld cracks, porosity, etc. It can also be ised for checking the finish and chamfer on interior "O" ring grooves and keyways. It is 3/4 inch in diameter and 4 inches long when fully extended and contains an objective lens that magnifies outward in a "v" shape configuration (fisheye lens). It can be used on holes from 3/16 to 5 inch diameter. The whole in- Chap 4 Par 404 Page 135
terior diameter will be in focus at all times. On larger diameters, the instrument would have to be tipped slightly to focus on a specific area of sidewall. The instrument is suitable for viewing parts from 1 inch to 20 feet in length. The focus can be adjusted by holding the instrument in one hand and turning the zoom with the other hand for sharper definition of any depth of field. The area is sharp and in focus at any given focal plane and will vary from about 6 inches to 5 feet, depending on the length of the part and distance from the instrument. For more detail beyond 5 feet, a magnification booster is available. The zooming borescope does not have its own light source, but relies on existing ambient light. Unless the part is very small in diameter, or standard room illumination is poor, no additional light source is necessary. should additional light be necessary, an inspection flashlight (see Table 3-8) can be held against the other end of the part.
FIGURE 4-13. ZOOM-A-BORE INSPECTION INSTRUMENT WITH
MAGNIFICATION BOOSTER ATTACHED (Courtesy of Titan Tool Supply Company, Inc.)
(8) Micro-Bore Viewing System. Small area borescope viewing systems exist for optical checking of bores and cavities (see Figure 4-14). This system does not fit the definition of a rigid borescope; however, it is used for inspection of bores and cavities. The system was designed to overcome the lighting limichap 4 Page 136 Par 404
tations of some of the micro-borescopes and will not break as easily as the small-diameter borescope probes. This optical device in the illustration comprises 17 different lenses on a rotary system, each with a predetermined focal length (minimum of
0.020 inch to a maximum of 10 inches). It can be used for the internal viewing of micro-bores, tubes, and cavities from 0.039 to 0.315 inch diameter, with depths up to 10 inches. The illumination system is adjustable and contains a 2.5 volt Halogen light bulb. For deep cavities, additional illumination may be necessary, which can be provided by a small diameter inspection light.
FIGURE 4-14. MICRO-BORE VIEWING SYSTEM (Courtesy of Titan Tool Supply Company, Inc.)
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The micro-bore viewing system can be used in aircraft maintenance for checking the inside of small tubes, miniature drilled, bored, and ground holes for surface finish, cracks, dirt, weld cracks, porosity, etc. It can also be used for inspecting the inside of miniature holes in turbine blades and valve bodies.
(9) Mirror Tube Viewing Systens. These systems allow the viewing of small holes and cavities and are meant to be inexpensive alternatives to a rigid borescope for some applications.
The system shown in Figure 4-15 allows the viewing of holes and cavities to a depth of 2 inches with 5-power magnification without any attachments. Included with the unit are two separate twist-on mirror tubes, a 0.196 inch diameter, with a viewing depth of 1 inch, and 0.236 inch diameter, which allows a viewing depth of 1.2 inches. Illumination is provided by a miniature Halogen light bulb projecting the illumination directly on the axis down the hole. Powered by two AA batteries.
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90° Viewing FIGURE 4-12' (courtesy of Titan Tool Supply MIRROR TUBE VIEWING SYSTEM Company, Inc.)
The mirror viewing system shown in Figure 4-16 is for viewing holes up to 68 inches in depth. The main probe can be 4 to 20 mm in diameter. The field of view is 30 to 70 degrees.
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AC 43—204 8/14/97 FIGURE 4-16. DEEP-HOLE MIRROR VIEWING SYSTEM (Courtesy of Instrument Technology, Inc.)
Flexible Borescopes. Flexible borescopes are generally used in applications that do not have a straight line access path to the area to be inspected. Some typical uses of flexible borescopes in aircraft maintenance are inspection of corrosion, Chap 4 Page 140 Par 404