cracks, and scratches on bore walls of castings; 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. The two types of flexible borescopes are flexible fiberscopes and flexible videoscopes.
Figure 4-17 shows some typical flaws seen through a flexible fiberscope D FIGURE 4-17. TYPICAL FLAWS SEEN THROUGH A FLEXIBLE FIBERSCOPE (Courtesy of Olympus Corporation) Chap 4 Par 404 Page 141
Flexible Fiberscopes. A typical flexible fiberscope consists of a fiber-optic light-guide bundle, a fiber-optic image-guide bundle, an objective lens, interchangeable viewing heads, and remote controls for articulation of the distal tip.
Flexible fiberscopes are available in diameters from 0.055 to 0.5 inch and in lengths of over 300 feet. The light-guide bundle is used to carry illumination from a remote source to the distal Since each fiber in the light-guide bundle transmits only illumination and not an image, the fibers do not have to be precisely aligned and are generally larger than those in the imageguide bundle. The image-guide bundle is used to carry the image formed by the objective lens back to the eyepiece, and the fibers in the image guide are precisely aligned so that they are in an identical relative position to each other at their terminations for proper image resolution. The diameter of the fibers in the image guide is a factor in obtaining good image resolution.
large number of small-diameter fibers in the image-guide will provide a bright image with high resolution (see Figure 4-18).
lich resolution allows the use of an obiective lens with a wider field of view and also allows magnification of the image at the eyepiece for better viewing of objects at the periphery of the image (see Figure 4-19). Interchangeable distal tips provide various directions and fields of view on a single fiberscope;
however, because the tip can be articulated for scanning purposes, distal tips with either a forward or side viewing direction are usually sufficient. Fields of view are typically 40 to 60 degrees, although they can range from 10 to 120 degrees. Most fiberscopes provide adjustable focusing of the objective lens.
Chap 4 Par 404 Page 142
8/14/97 AC 43-204 Two views down a combustar con with the datal tip in the some pastion. A fiberscope with smaller diameter fibers and 40% more fibers in the image bundle provides better resolution (a) than a fiberscope Riben (b). Courtesy of Olympus Corporation FIGURE 4-18. COMPARISON VIEWS WITH DIFFERENT QUANTITIES OF
FIBERS IN THE FIBERSCOPE IMAGE BUNDLE (Courtesy of Olympus Corporation) chap 4 Par 404 Page 143
8/14/97 AC 43-204 ENGINE SN • 709100 * USAIR * J180 NEV CAN • 7 08/84/87 12:20:09 ENCINE SN # 709100 * USAIR * TTBD FUEL NOZZLE 08/04/87 12:58:25 0:19:05 FIGURE 4-19. FLEXIBLE VIDEOSCOPE IMAGES (Courtesy of Welch Allyn, Inc.)
Chap 4 Page 144 Par 404
Flexible Videoscopes With CCD Probes. Flexible videoscopes have CCD imaging sensors at the distal tip to electronically transmit color or black and white images to a video moni- The fiber-optic image bundle inside the insertion tube is replaced by a wire assembly to carry video information. Videoscopes with CCD imaging sensors produce images with spatial resolutions on the order of those described in Figure 4-20.
90° FIELD OF VIEW 60° FIELD OF VIEW 30> FIELD OF VIEW UMM LIN. UMM LIM. UMM LIN.
102 T - USEABLE 4020 USEABLE ISEABLE - RESOLUTION 300 RESOLUTION 300 RESOLUTION 300 200 200 100 100
• ~ 20 80 100 40 6D 10%
OBJECT DISTANCE MM OBJECT DISTANCE MM OBJECT DISTANCE MM WIN. = Lines per inch UMM = Lines per millimeter SHARP FOCUS FIGURE 4-20. TYPICAL CCD VIDEOSCOPE RESOLUTION (Courtesy of Welch Allyn, Inc.)
The resolution of videoscopes, like rigid borescopes and flexible fiberscopes, depends on the object-to-lens distance and the field of view, since these two factors affect the amount of magnification. Generally, videoscopes produce higher resolution than fiberscopes, although riberscopes with smaller diameter fibers can be competitive with the resolution of videoscopes. A primary advantage of videoscopes is their longer working length. with a given amount of illumination at the distal tip, videoscopes can return an image over a greater length than fiberscopes.
features of videoscopes include (1) the display can help reduce eye fatigue (but does not allow the capability of direct viewing through an eyepiece); (2) there is no honeycomb pattern or irregular picture distortion as with some fiberscopes (see Figure 4-21); (3) the electrical image signal allows digital image enhancement and the potential for integration with automatic inspection systems; and (4) the display allows the generation of reticles on the viewing screen for point-to-point measurements.
Chap 4 Par 404 Page 145
• FIGURE 4-21. COMPARISON BETWEEN VIDEOSCOPE AND FIBERSCOPE IMAGES (Courtesy of Welsh Allyn, Inc.)
Chap 4 Page 146 Par 404
Borescopes With Special Features. Borescopes, fiberscopes, video systems, and related equipment are being used for more purposes, more complex inspections, and even some limited repair applications. Some of the borescope products briefly described are measuring borescopes and fiberscopes, working channels, flying probe, VideoHook Probe, power blending borescope, and ShadowProbe™.
(1) Measuring Borescopes and Fiberscopes. Measuring borescopes and fiberscopes contain a moveable cursor that allows measurements during viewing (see Figure 4-22). When the object under measurement is in focus, the movable cursor provides a reference for dimensional measurements in the optical plane of the This capability eliminates the need to know the objectto-lens distance when determining magnification factors. Measuring borescopes and fiberscopes can be used in aircraft maintenance for measuring cracks in aircraft turbine liners, blade damage, etc.
FIGURE 4-22. VIEW THROUGH A MEASURING FIBERSCOPE (Courtesy of Olympus Corporation) Working Channels. Working channels are used in borescopes and fiberscopes to pass working devices to the distal Figure 4-23 shows a retrieval tool in a working channel picking up a screw inside a machine. Table 4-3 shows some typical devices for use in working channels. working channels are presently used to pass measuring instruments, retrieval devices, and hooks for aiding the insertion of thin, flexible fiberscopes.
working channels are used in flexible fiberscopes with diameters Chap 4 Par 404 Page 147
as small as 0.106 inch. Working channels are also under consideration for the application and removal of dye penetrants and for the passage of wires and sensors in eddy current measurements.
Retrieval tools are typically used in aircraft maintenance for removing objects from aircraft turbines and appliances that may have been dropped during maintenance and repair work.
FIGURE 4-23. RETRIEVAL DEVICE IN BORESCOPE WORKING CHANNEL (Courtesy of Olympus Corporation) Chap 4 Page 148 Par 404
TABLE 4-3. TYPICAL WORKING CHANNEL DEVICES (Sheet 1 of 2) (Courtesy of Olympus Corporation)
TYPE DIAGRAM COMMENT
SAMPLING FORCEPS Standard Round jaws for normal grasping.
(Fenestrated 20 . Ellipsoid Longer jaws in order to grasp more (Fenestrated) than the standard type.
3. Alligator Designed for scooping with side opening jaws.
4. Crescent For lassoing foreign objects.
5. Hexagonal For lassoing foreign objects.
6. Oval For lassoing foreign objects.
B. GRASPING FORCEPS Tripod Good for gripping larger solid ob- Jects
2. Hooked Jaws Good for clenching into softer objects.
3. "W" Shaped collects flat foreign objects,
e.g., coins.
4. Alligator Jaws Designed to prevent slipping, grip well.
5. Rat Tooth Jaws hinge together to provide good grip.
6. Pelican Type Useful for awkward foreign objects.
7. Basket Type Basket tightens up on an object for removal, e.g., ball bearings.
chap 4 Par 404 Page 149
TABLE 4-3. TYPICAL WORKING CHANNEL DEVICES (Sheet 2 of 2) (Courtesy of Olympus Corporation) B. GRASPING FORCEPS (Continued)
8. Rubber Tipped Jaws Good for grasping smooth surfaces.
c. MAGNETIC EXTRACTORS Cylindrical Shape Good for retrieving ferromagnetic foreign objects.
(3) Flying Probe. In the so called Flying Probe, a flexible video borescope uses an air jet microthruster to help position a flexible video probe in an area to be viewed (see Figure 4-24). The air jet force can be used to pull the distal tip of the flexible video borescope through cavities to locations previously unreachable by traditional borescope techniques. The air jet microthruster system is particularly suitable for aircraft engine and structural inspections that would otherwise require the use of guide tubes.
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8/14/97 AC 43-204 FIGURE 4-24. FLYING PROBE FLEXIBLE BORESCOPE (Courtesy of Welch Allyn, Inc.)
(4) VideoHook Probe. This probe consists of a special hook attachment at the distal tip of a flexible video probe (see Figure 4-25). The special hook attachment can be hooked onto the first stage turbine blade of a Pratt & Whitney JT8D series engine for more complete viewing of the second-stage, high-pressure turbine vanes. After hook attachment, the engine rotor is slowly rotated manually to allow viewing of the full second-stage vanes.
The hook attachment can be slipped onto the distal tip of the Chap 4 Par 404 Page 151
VideoHook Probe, with no special tools or working channel required, and can be easily removed when the inspection is completed. The hook is made of special soft aluminum that will reduce damage should it be left behind in the engine.
FIGURE 4-25. VIDEOHOOK PROBE ATTACHMENT FOR A FLEXIBLE BORESCOPE (Courtesy of Welch Allyn, Inc.)
(5) Power Blending Borescope Kit. A combined borescope and blending system can detect and repair (i.e., blend) foreign object damage on the 7th-stage compressor blades of JT8D-200 series engines (see Figure 4-26). The power blending borescope allows engine repair with the engine installed on the aircraft and without engine disassembly. The power blending procedure is ac- Chap 4 Page 152 Par 404