Repairing Facing and Core Damage

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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flows freely from the drilled holes. After the voids are completely tilled, bring the area down to proper thickness by working the excess resin out through the holes, and then cure and refinish. Repairing Facing and Core Damage The repair of facings and cores requires more than one method of repair. Special attention must be given to the type of core used. HONEYCOMB CORE.—The repair of facings and cores requires more than one method of repair. Special attention must be given to the type of core used. Damages extending completely through one facing of the material and into the core require removal of the damaged core and replacement of the damaged facings in such a manner that normal stresses can be carried over the area. The scarfed method, illustrated in figure 14-11, is the preferred method for accomplishing small repairs of this type. Repairs of this type maybe accomplished as follows: Carefully trim out the damaged portion to a circular or oval shape and remove the core completely to the opposite facing. Be careful not to damage the opposite facing. The damaged facing around the trimmed hole is then scarfed back carefully by sanding. The length of the scarf should be at least 100 times the facing thickness, as shown in view B of figure 14-11. This scarfing operation must be done very accurately to a uniform taper. Cut a piece of replacement core material (or a suitable substitute) to fit snugly in the trimmed hole. It should be equal in thickness to the original core material. Brush coat the repair area and the replacement honeycomb, exercising care to prevent an excessive amount of resin from entering the honeycomb cells. Insert the honeycomb repair section and place the resin-impregnated cloth over the repair area, as shown in view C of figure 14-11. Cover the repair area with cellophane sheeting, and cure the repair in accordance with the resin manufacturer’s instructions. After the repair has been cured, sand the surface to its original contour. The entire area should be lightly sanded before refinishing. FOAM CORE.—The damaged core should be removed by cutting perpendicular to the surface of the face laminate opposite the damaged face. Scrape the inner facing surface clean, making sure there is no oil or grease film in the area, to ensure good bondage of the foam to the laminate. Fill the area where the core has been removed with the tiller material specified in the aircraft structural repair manual. Figure 14-12 shows the replacement of a foam core. NOTE: Do not use MEK to clean the damage as it may soften and weaken the foam. Repairing Puncture Damage The repair of punctures differs as to the method used. Repair of honeycomb cores is different than the repair of foam cores. Figure 14-11.—Honeycomb-type core repair. 14-10

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