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TYPES OF REPAIRS
FLUSH PATCH CLEAR OF INTERNAL STRUCTURES

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure 13-58.—Lap patch for repairing a crack in stressed skin. To repair cracks, you should always drill a small hole (normally called stop drilling) in each end of the crack before applying the patch. This is normal] y done by using a No. 30 or No. 40 drill bit. This prevents the concentration of stresses at the apex of the crack and distributes the stresses around the circumference of the hole. The patch must be large enough to install the required number of rivets as determined from the rivet schedule indicated for the gauge material in the area that is damaged. See figure 13-58. The recommended patch may be cut in a circular, square, rectangular, or diamond shape. The edges are normally chamfered (beveled) to an angle of 45 degrees for approximately one-half its thickness. The rivet pattern is laid out on the patch by using the proper edge distance and spacing. The installation position of each rivet is marked with a center punch. The impression in the material made with the center punch helps to keep the drill from slipping away from the hole being drilled. See figure 13-59. Drill only a minimum number of rivet holes in the patch; normally four will suffice at an angle of 90 degrees to each other. Position the patch over the surface being repaired, and ensure that the correct edge distances are being maintained. Drill four holes in the surface being repaired, using the predrilled holes in the patch as a pattern for alignment. As each hole is drilled, using the proper temporary fasteners, secure the patch in place. When the patch is temporarily secured, drill the remaining rivet holes through the patch and the surface being repaired. Remove the patch and deburr all rivets holes with a deburring tool or a large drill bit. Prime the repair materials with the proper corrosion-preventive material before the riveting operation. Secure the patch in position with temporary fasteners to maintain alignment during riveting. Riveting procedures were covered earlier in this chapter. Holes may be repaired in either stressed or nonstressed skin that is less than three-sixteenths of an inch in diameter by filling with a rivet. Drill the hole and install the proper size rivet to fill the hole. For holes three-sixteenths of an inch and larger, you should consult the applicable structural repair manual for the necessary repair information. The damaged area is removed by cutting and trimming the hole to a circular, square, rectangular, or diamond shape. The corners of the hole should be rounded to a minimum of one-fourth of an inch in radius. The lap patch is fabricated and installed in the same manner as previously explained for repairing cracks. Flush Patches A flush patch consists of a filler patch that is flush with the skin after it is inserted. It is backed up and riveted to a reinforcement plate that, in turn, is riveted to the inside of the skin. This reinforce- ment plate is usually referred to on some repair diagrams as the doubler or the backup plate. On some high-performance aircraft, only the flush patch is permitted in making skin repairs. Flush patches should be used where aerodynamic smoothness is required. The type of flush patch used depends on the location of the damaged area. One type is clear of internal structures, and the other is not. Like all types of repairs, you must consult the applicable structural repair manual for the necessary Figure 13-59.—Drilling holes for rivets. 13-43







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