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.

repair information. The repairs discussed next are typical of most repairs.

FLUSH PATCH CLEAR OF INTERNAL STRUCTURES. In areas that are clear of internal structure, the repair is relatively simple to make. This is especially true where there is an access door or plate through which the rivets can be bucked. In inaccessible areas, the flush patch may be made by substituting blind rivets for standard rivets, where permissible, and devising a means of inserting the doubler through the opening.

One method, shown in figure 13-60, has a doubler that has been split. To insert the doubler, slip the split edge under the skin and twist the doubler until it slides in place under the skin. The screw in the center of the doubler is temporarily installed to serve as a handle for inserting the doubler through the hole. This type of patch is normally recommended for holes up to 1 1/2 inches in diameter. In holes larger than 1 1/2 inches, trim a hole to a rectangular or elliptical shape and round the corners to a generous radius. See figure 13-61.

Figure 13-61.-Flush rectangular patch.

On larger repair areas, it is usually possible to buck the doubler rivets by inserting and holding the bucking bar through the center of the doubler. The filler is then riveted in place with blind fasteners. When blind rivets are used as substitutes for solid rivets, the structural repair manual normally specifies the next larger size. The proper edge distances for the substitute fasteners must be maintained.

NOTE: Edge distance was discussed earlier. In all flushpatches, the filler should be of the same gauge and material as the original skin. The doubler, generally, should be of the same material and one gauge heavier than the skin. Structural repair manuals will specify the allowable substitution of materials. This can be in the form of a note on the repair diagram. When you are laying out the size of the doubler, the length should exceed the width. This enables the doubler to be slipped in through the skin and positioned for installation. This eliminates the splitting and manipulation of the patch required in installing doublers of square and round flush patch repairs.

The filler is fabricated slightly less than the dimensions of the hole being repaired. Generally, the maximum clearance between the skin and the filler is one thirty-second of an inch. This will allow a 1/64-inch clearance on each end of the filler and eliminate any possibility of stress developing from contact between the two parts. The doubler is fabricated larger than the hole being repaired to allow for the specified number of rivets required to attach the doubler to the skin being repaired. The doubler, filler, and attaching skin rivet pattern may be laid out, drilled, and deburred in the identical manner as described for a lap patch. After the required corrosion-preventive materials have been applied, the doubler is positioned in the structures interior and secured with temporary fasteners. Inspect the rivet holes for proper alignment, and rivet the doubler in place with solid rivets. The filler can then be riveted in place with blind fasteners.

NOTE: If the flush repair is in an open skin area, the filler may be riveted to the doubler prior to installing the doubler.