Flush patch over internal structures

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Flush Patches

Aviation Structural Mechanic

Skin replacement

FLUSH PATCH OVER INTERNAL STRUCTURES. Fabricating a flush patch over internal structures may become difficult. In some instances, it may be done simply with a split doubler and a filler, as shown in figure 13-62. Frequently a split doubler, filler strips, and filler are used in the repair. The filler strip is used as a spacer if a structural component under the skin has been damaged. In all cases, the existing structures rivet holes should be used when the rivet pattern is laid out. The flush patch over internal structure is installed with the same methods as described for a flush patch

Figure 13-62.Flush patch over internal structure.

clear of internal structure, except for modification of the doubler.

CHEM-MILLED SKIN REPAIR. On some aircraft the fuselage skin is Chem-Milled or machine tapered and highly stressed. Figure 13-63 shows a Chem-Milled skin repair in a pressurized fuselage section. The skin consists of a shim, a doubler, and a filler. The damage area is trimmed and the inside corners are filed to one-fourth of an inch in radius. The replaced metal and rivets or other fasteners must be equal to or stronger than the original. The structural repair manual should be consulted for fastener spacing, edge distance, and repair procedure. During final assembly of the repair, the fabricated parts should be bonded together with an adhesive to ensure pressurization is maintained.

FLUSH ACCESS DOOR. A flush access door installation, as shown in figure 13-64, is sometimes permitted. It is installed to make repair to the internal structure easier and to permit repair of damage to the skin in certain areas. The flush access door consists of a doubler and a stressed cover plate. The cover plate is normally fabricated from material identical to the skin. A single row of nut plates is riveted to the doubler. The doubler is then riveted to the interior side of the skin with two rows of rivets, staggered as shown in figure 13-64. The cover plate is attached to the doubler with machine screws. When an access

Figure 13-63.-Chem-Milled skin repair (fuselage pressurized).

door is permitted and installed over the internal structure, screws should be installed through the cover plate into the internal structural member wherever possible.