Page 3 2
nd
Edition – Rev 0
51-20
DAMAGE CLASSIFICATION AND REPAIRS - GENERAL
Herein is reported the typical kinds of defects:
Dent: a dent is depressed or hollow deformation without removal of material or change
in cross sectional area (see scratch & crack definition below). Stress wrinkles caused
by failure of a fastener are not considered a dent. Generally dents are caused by impact
from a smoothly contoured object. One characteristic that all dents should have is a
"pushed in surface" and a relatively smooth bottom where metal is not displaced,
folded or creased. Generally when evaluating dents, the width of the dent is the second
longest distance across the dent, measured at 90 degrees to the direction of the length.
Nicks: broken edges without cracks, but with portions of material removed.
Scratches: marks penetrating the surface that reduce the structural cross section of the
material but do not penetrate the complete thickness. The depth of a scratch may be
determined by use of an optical micrometer.
Cracks: fractures that would not separate the material into two parts if the surrounding
supports were removed; usually originating at edges, holes, or points where
concentrated loads are applied or where abrupt changes in cross-sectional area occur.
Cracks cause a significant cross-sectional area change. This damage usually has an
irregular line and is often the result of fatigue in the material. The length of cracks that
may be tolerated varies widely with material, structure, and application. No crack
should be regarded as negligible until the damage limits for the affected structure have
been determined. True crack length determination will generally require some form of
Non Destructive Testing such as Eddy Current or Fluorescent penetrants.
Holes: punctures penetrations or cutouts that breach the complete thickness of the
material and is fully surrounded by undamaged material. The size, shape, and distance
from edges and supporting structures must be considered when evaluating hole damage.
Abrasion: a damaged area that is the result of scuffing, rubbing, scraping, or other
surface erosion. This type of damage is usually rough and has an irregular shape.
Gouge: a damaged area where the result is a cross-sectional change caused by a sharp
object and gives a continuous, sharp or smooth groove in the material
Corrosion: deterioration of a metal because of an electrochemical reaction with its
environment. Depending on the type of corrosion, this deterioration may take the form
cracking, exfoliation, or erosion of the corroding material. Corrosion damage is
typically classified as light, moderate, or severe, depending on the extent of the
corrosion and the loading requirements of the corroded part.
It is important to keep the aircraft clean and to remove any collection of
corrosive agents such as oil, grease, dregs and other foreign matter. To avoid
damage to finish, do not use polishing detergents. Original or equivalent
corrosion prevention must be re-applied after any alteration or repair.If any
trace of corrosion is detected it should be removed as soon as possible and part
should be immediately treated to prevent further corrosion.
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