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Summary

Skin patches are divided into two types. The first, a lap patch, is applied externally with its edges overlapping the skin. This patch is used only where an aerodynamic surface is not needed. Flush patches, the second type, are inserted into the damaged area and riveted to a doubler that is riveted to the skin. Damaged-area accessibility and directions in the technical manual applicable to the aircraft determine the kind of patch to use. Most of an aircraft's skin is inaccessible from the inside, and such skin is designated as closed skin. That accessible from both sides is open skin.

An important aspect of making skin repairs is the amount of stress intensity (strength) to be restored. Repairs must restore highly critical areas to 100 percent, semicritical areas to 80 percent, and noncritical areas to 60 percent of original strength. For stressed-skin repairs, the applicable technical manual must be consulted and followed.

Lap patches are used at authorized locations to repair cracks and small holes. To repair cracks, a small hole is drilled at both ends of the crack to stop the crack from spreading. Patch sizes must be large enough to take the number of rivets prescribed in the applicable manual. Flush plates are relatively easy to install in locations free of internal structures. In inaccessible areas, blind rivets are substituted for standard ones. With these patches the doubler is usually split and inserted or rotated into position. In flush patches, the filler must be of the same gage and material as the original skin, and the doubler generally must be one gage heavier.

Repairs on hulls and floats must be watertight, and pressurized compartment repairs must be sealed against pressure loss. Whenever possible, the manufacturer's sealing method must be used. A recommended method includes cleaning contact surfaces, removing any burrs and chips, using zinc-chromate tape and paste, and bringing the mating surfaces together with machine screws and nuts to complete the seal.

Installing flush access doors is an easy and efficient way to repair internal structures and some skin areas. Such doors consist of a doubler and a stressed access coverplate. The doubler is riveted to the skin, and the plate is attached to the doubler with nut plates and machine screws.

The number of patches or repairs can be so numerous or the skin damage so extensive that panel replacement is required. Inspecting such areas includes a careful examination of internal structures. Bent, fractured, or wrinkled members must be repaired or replaced. Rivets in the vicinity must be inspected for failure. The technical manual for the particular aircraft must be consulted to determine the rivet inspection procedure.

The gage and alloy for the replacement panel is shown in the applicable manual for the specific aircraft. Either of two ways can be used to determine the new panel's size. Dimensions can be measured during inspection, or the old skin can be used as a template. The latter method is more accurate and is preferred. The new sheet is drilled near the center using the holes in the old sheet as a guide. The two sheets are held together with sheet metal fasteners as the holes are drilled outward from the center. Lacking a template the holes in the reinforcing members are used as guides to drill the new holes. Sheet metal fasteners are used here as with a template. Because most ribs, stringers, and bulkheads depend on the skin for some of their rigidity, duplicating holes from the frame or reinforcing members must be done with care. Drilling pressure can force the skin away from the frame and make the holes out of alignment. A wood block held firmly against the skin while drilling can prevent this. Unless the drill is held at a 90° angle, the holes will be out-of-round. Where a straight drill cannot be inserted, an angle attachment or flexible-shaft drill is necessary. New hole locations can be marked with a soft pencil, a prick punch, or a hole finder.

A straight bucking bar is preferred because its weight can be applied in a direct line with the rivet's shaft. Where internal structural members prevent using a straight bar, one allowing straightline pressure must be used.

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