Inspect the glue line using a magnifying glass. Where the glue  line tends to part, or where the presence of glue cannot be  detected or is suspect, probe the glue line with a thin feeler  gauge. If any penetration is observed, the joint is defective.  The structure usually dictates the feeler gauge thickness,  but use the thinnest feeler gauge whenever possible. The  illustration indicates the points a feeler gauge should  probe.  [Figure 6-6] Pressure exerted on a joint either by the surrounding structure  or by metal attachment devices, such as bolts or screws, can  cause a false appearance of the glue condition. The joint must  be relieved of this pressure before the glue line inspection  is performed. A glued joint may fail in service as a result of an accident or  because of excessive mechanical loads having been imposed  upon it. Glued joints are generally designed to take shear  loads. If a joint is expected to take tension loads, it is secured  by a number of bolts or screws in the area of tension loading.  In all cases of glued joint failure, whatever the direction of  loading, there should be a fine layer of wood fibers adhering  to the glue. The presence of fibers usually indicates that the  joint itself is not at fault. Examination of the glue under magnification that does not  reveal any wood fibers, but shows an imprint of the wood  grain, indicates that the cause of the failure was the predrying  of the glue before applying pressure during the manufacture  of the joint. If the glue exhibits an irregular appearance with  star-shaped patterns, this is an indication that precuring of the  glue occurred before pressure was applied, or that pressure had been incorrectly applied or maintained on the joint. If  there is no evidence of wood fiber adhesion, there may also  be glue deterioration. Wood decay and dry rot are usually easy to detect. Decay  may be evident as either a discoloration or a softening of  the wood. Dry rot is a term loosely applied to many types  of decay, but especially to a condition that, in an advanced  stage, permits the wood to be crushed to a dry powder. The  term is actually a misnomer for any decay, since all fungi  require considerable moisture for growth. Dark discolorations of the wood or gray stains running  along the grain are indicative of water penetration. If such  discoloration cannot be removed by light scraping, replace  the part. Disregard local staining of the wood by dye from a  synthetic adhesive hardener.  In some instances where water penetration is suspected, a  few screws removed from the area in question reveal, by  their degree of corrosion, the condition of the surrounding  joint.  [Figure 6-7]  Another method of detecting water penetration is to remove  the bolts holding the fittings at spar root-end joints, aileron  hinge brackets, etc. Corrosion on the surface of such bolts  and wood discoloration provide a useful indication of water  penetration. Plain brass screws are normally used for reinforcing glued  wooden members. For hardwoods, such as mahogany or ash,  steel screws may be used. Unless specified by the aircraft  Wood Condition  Shrinkage Shrinkage Ply spar web Inspection hole in web Metal fitting Bolt A A A A A A At all points marked , check  for glue condition and separation with a feeler gauge. Laminated spar Figure 6-6.  Inspection points for laminated glue joints. 6-5