means, as would occur with the use of a saw blade,  chisel, or sharp-edged stone striking a glancing blow. Negligible damage consists of visually apparent, surface  damage that do not affect the structural integrity of the  component involved. Negligible damage may be left as is or  may be corrected by a simple procedure without restricting  flight. In both cases, some corrective action must be taken to  keep the damage from spreading. Negligible or minor damage  areas must be inspected frequently to ensure the damage does  not spread. Permissible limits for negligible damage vary  for different components of different aircraft and should be  carefully researched on an individual basis. Failure to ensure  that damages within the specified limit of negligible damage  may result in insufficient structural strength of the affected  support member for critical flight conditions. Small dents, scratches, cracks, and holes that can be repaired  by smoothing, sanding, stop drilling, or hammering out, or  otherwise repaired without the use of additional materials,  fall in this classification.  [Figure 4-170] Damage repairable by patching is any damage exceeding  negligible damage limits that can be repaired by installing  splice members to bridge the damaged portion of a structural  part. The splice members are designed to span the damaged  areas and to overlap the existing undamaged surrounding  structure. The splice or patch material used in internal riveted  and bolted repairs is normally the same type of material as  the damaged part, but one gauge heavier. In a patch repair,  filler plates of the same gauge and type of material as that in  the damaged component may be used for bearing purposes or  to return the damaged part to its original contour. Structural  fasteners are applied to members and the surrounding  structure to restore the original load-carrying characteristics  Damage Repairable by Patching Negligible Damage •  Dent—indentation in a metal surface produced by an  object striking with force. The surface surrounding  the indentation is usually slightly upset. •  Erosion—loss of metal from the surface by  mechanical action of foreign objects, such as grit or  fine sand. The eroded area is rough and may be lined  in the direction in which the foreign material moved  relative to the surface. •  Chattering—breakdown or deterioration of metal  surface by vibratory or chattering action. Although  chattering may give the general appearance of metal  loss or surface cracking, usually, neither has occurred. •  Galling—breakdown (or build-up) of metal surfaces  due to excessive friction between two parts having  relative motion. Particles of the softer metal are torn  loose and welded to the harder metal. •  Gouge—groove in, or breakdown of, a metal surface  from contact with foreign material under heavy  pressure. Usually it indicates metal loss but may be  largely the displacement of material. •  Inclusion—presence of foreign or extraneous material  wholly within a portion of metal. Such material is  introduced during the manufacture of rod, bar or  tubing by rolling or forging. •  Nick—local break or notch on an edge. Usually it  involves the displacement of metal rather than loss. •  Pitting—sharp, localized breakdown (small, deep  cavity) of metal surface, usually with defined edges. •  Scratch—slight tear or break in metal surface from  light, momentary contact by foreign material. •  Score—deeper (than scratch) tear or break in metal  surface from contact under pressure. May show  discoloration from temperature produced by friction. •  Stain—a change in color, locally causing a noticeably  different appearance from the surrounding area. •  Upsetting—a displacement of material beyond the  normal contour or surface (a local bulge or bump).  Usually it indicates no metal loss. Damages may be grouped into four general classes. In many  cases, the availabilities of repair materials and time are the  most important factors in determining if a part should be  repaired or replaced. Classification of Damage Figure 4-170.  Repair of cracks by stop-drilling. Crack Stop-drill cracks 4-91