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 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.