becomes brittle around the damaged area as a result of the heat, and minute cracks are created by the impact of the projectile. These cracks open up under vibration. If the projectile passes through the component or structure, it will leave a larger hole on the opposite side from where it entered. The repair procedures for combat damage should be followed with extreme care only after a rigid inspection of the damage has been completed in accordance with the General Manual for Aircraft Battle Damage Repair, NAVAIR 01-1A-39. HEAT.—Certain areas of high-performance air- craft are exposed to high temperatures. These areas usually include the engine bleed lines, fuselage sections around the engine, the aft fuselage and horizontal stabilizer, and the wing sections around the boundary layer control system. Some aircraft structural repair manuals include diagrams that illustrate the heat danger areas. STRESS.—This type of damage is usually identi- fied by loosened, sheared, or popped rivets; wrinkled skin or webs; and cracked or deformed structural members. This damage is usually caused by violent maneuvers or hard landings. When the pilot reports these discrepancies on the yellow sheet, a thorough inspection of the entire aircraft must be performed. Investigation of Damage There are three methods that can be used to ensure a thorough investigation has been made. The three methods are visual inspection, hardness testing, and nondestructive inspection for cracks. VISUAL INSPECTION.—A thorough inspec- tion of the structure should be made for dents, scratches, abrasions, punctures, cracks, distortion, loose joints, breaks, and buckled or wrinkled skin. All riveted and bolted joints in the vicinity of the damaged area should be checked for elongated holes and loose, sheared, or damaged rivets or bolts. If any doubt exists about the failure of a rivet or bolt, the fastener should be removed for a more thorough inspection. All access panels, hatches, and doors should be opened to inspect the internal structure. A borescope (precision optical instrument) can be used for the inspection of the internal structure. By using this instrument, areas may be examined without being disassembled. You can view the area through the eyepiece. The adjacent structure should be inspected to determine if secondary damage has resulted from the transmission of shock or the load that caused the primary damage. A shock at one end of a structural member may be transmitted to the opposite end of the member and cause rivets to shear or other damage. When you estimate the extent of damage, be sure that no secondary damage remains unnoticed. Every precaution must be taken during the inspection to ensure that all corrosion is detected, especially in places where it will not be visible after repair. Past experience has proven that corrosion occurs more often in parts of the structure that are poorly ventilated and in inaccessible corners of internal joints that prevent proper water drainage. HARDNESS TESTING.—When fire has damaged the airframe, the paint will be blistered or scorched and the metal will be discolored. When these conditions exist, the affected area should first be cleaned and the paint removed. Following this, a hardness test should be conducted to determine if the metal has lost any of its strength characteristics. This test can be performed with the Barcol or Riehle portable hardness tester (described in chapter 1 of this manual). If the material to be tested is removed from the airframe, then a more reliable test can be made by using a standard bench tester (also described in chapter 1). If the alloy to be tested is either clad or anodized, the surface coating must be removed to the bare metal at the point of penetrator contact. This is necessary because clad surfaces are softer and anodized surfaces are harder than the base alloy. INSPECTION FOR CRACKS.—The existence of suspected cracks or the full extent of apparent cracks in structural members cannot be accurately determined by visual inspection. In cases where it is necessary for cracks to be accurately defined, a nondestructive inspection is usually performed. Fittings should receive a special investigation if they are cracked, since this could cause an entire component to fail. Fittings are used to attach sections of wings together and wings to fuselage, as well as attachment of stabilizers, control surfaces, landing gear, and engine mounts. The penetrant method of 13-37