inspection can be used to detect surface cracks in fittings and the magnetic particle method used to detect subsurface cracks in ferrous fittings. Nondestructive inspection of metals is described in chapter 15. CLEANUP OF DAMAGE.—Along with the investigation of damage, you should clean all jagged holes, tears, or damaged material. The cleaned sections must include all the area in which minute cracks are present. The affected area must be cut and rounded to form a smooth regular outline. If a rectangular- or square-shaped cutout is made, the radii for the corners should be a minimum of one-fourth inch, unless otherwise specified. All burrs should be removed from the edges of the cutout. All dented plates should be restored to their original shape if possible. Shallow abrasions or scratches should be burnished with a burnishing tool that will compress the projecting metal along the edges down into the scratch. Burnishing has no cutting action and removes no metal. When surface irregularities are smoothed by burnishing, the stress concentration will be lessened. NOTE: Deep scratches and abrasions must be treated as complete breaks. Classification of Damage After the extent of damage has been determined, it should be classified in one of the following categories: negligible damage, damage repairable by patching, damage repairable by insertion, or damage requiring replacement of parts. See figure 13-56. Before proceeding with the repair of the airframe, it is necessary that the applicable structural repair manual be consulted for the procedures and materials to be used. If the applicable manual is not available, the General Manual for Structural Repair, NA 01-1A-1, may be used. If any conflict should exist between the two manuals, the specific manual takes precedence. NEGLIGIBLE DAMAGE.—Negligible damage is that damage or distortion that may be allowed to exist as is or corrected by some simple procedure, such as removing dents, stop-drilling cracks, burnishing scratches or abrasions, without placing a restriction on the flight status of the aircraft. Before classifying damage as negligible, make sure the damage complies with the manufacturer’s specified limits of negligible damage. DAMAGE REPAIRABLE BY PATCHING.— Damage that can be repaired by installing a reinforcement or patch to bridge the damaged portion of a part may be classified as a damage repairable by patching. Reinforcement members are attached to the undamaged portions of the part to restore full load-carrying characteristics and airworthiness of the aircraft. Damage repairable by patching is specified for each member of the airframe. DAMAGE REPAIRABLE BY INSERTION.— Damage that is extensive enough to involve a major portion of a member, but which is not so extensive as to require replacement, is classified as damage repairable by insertion. The repair is made by inserting a new section and splicing it to the affected member. DAMAGE REQUIRING REPLACEMENT.— Damage that cannot be repaired by any practical means is classified as damage requiring replacement. Short structural members usually must be replaced because repair of such members is generally impractical. DAMAGE REPAIR PROCEDURES Damage repair procedures vary greatly from aircraft to aircraft and the type of repair that is going to be performed. Also consult the applicable aircraft MIMs and the applicable aircraft structural repair manual before performing any structural repairs. Selection of Repair Material The major requirement in making a repair is the duplication of strength of the original structure. You should consult the structural repair manual for the aircraft concerned for the alloy thickness and temper designation of the repair material to be used. This manual will also designate the type and spacing of rivets or fasteners to be used in the repair. In some instances, substitutions of materials are allowed. When you are making a substitution of materials and conflicting information between manuals exists, the structural repair manual for the aircraft being repaired should be used. 13-38