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REPAIR CRITERIA
HAZARDS AND SAFETY PRECAUTIONS

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure 14-27.—Repair zones. critical zones include leading edges of wings and tails, forward nacelles and inlet areas, forward fuselages, and overwing areas of the fuselage. The least critical zones include trailing edges and aft fuselage areas. Repair Tools Drill motors should be capable of speeds of 2,000 to 5,000 rpm. These drills should be equipped with feed rate limiting surge controls to prevent backside breakout caused by feeding the drill too fast and excessive heat buildup from feeding the drill too slow. Feed rates should not exceed 30 seconds per inch, with 10 to 15 seconds per inch producing the best results on graphite-epoxy composites. The drill should be turning full speed prior to surface contact and during withdrawal from completed holes. These holes should be drilled slightly undersize and reamed to the required size.    The various types of drill bits used for drilling composites are either twist, flat fluted/spade/dagger, single flute, or piloted countersink, and they are made out of carbide or carbon steel. A drill stop (fig. 14-28) is an adjustable spring damper that is attached to the drill bit shank. This mechanically stops the drill at a predetermined depth prior to exiting the material backside, thus reducing backside breakout caused by the follow through. Firm pressure is required to overcome this spring tension for the drill to penetrate the laminates backside. Routers are high-speed, hand-held, portable cutters used for removing damaged skin or core materials. They are designed to operate on shop air at Figure 14-28.—Drill stop. 14-27







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