Splits in the fabric covering on plywood surfaces must be  investigated to ascertain whether the plywood skin beneath  is serviceable. In all cases, remove the fabric and inspect the  plywood, since it is common for a split in the plywood skin  to initiate a similar defect in the protective fabric covering. Although a preliminary inspection of the external structure  can be useful in assessing the general condition of the aircraft,  note that wood and glue deterioration can often take place  inside a structure without any external indications. Where  moisture can enter a structure, it seeks the lowest point, where  it stagnates and promotes rapid deterioration. A musty or  moldy odor apparent as you remove the access panels during  the initial inspection is a good indication of moisture, fungal  growth, and possible decay. Glue failure and wood deterioration are often closely  related, and the inspection of glued joints must include an  examination of the adjacent wood structure. NOTE: Water  need not be present for glue deterioration to take place.  The inspection of a complete aircraft for glue or wood  deterioration requires scrutiny of parts of the structure  that may be known, or suspected, trouble spots. In many  instances, these areas are boxed in or otherwise inaccessible.  Considerable dismantling may be required. It may be  necessary to cut access holes in some of the structures to  facilitate the inspection. Do such work only in accordance  with approved drawings or instructions in the maintenance  manual for the aircraft concerned. If drawings and manuals  are not available, engineering review may be required before  cutting access holes. The inspection of glued joints in wooden aircraft structures  presents considerable difficulties. Even where access to the  joint exists, it is still difficult to positively assess the integrity  of the joint. Keep this in mind when inspecting any glue joint. Some common factors in premature glue deterioration  include:  An aircraft painted in darker colors experiences higher  skin temperatures and heat buildup within its structure.  Perform a more detailed inspection on a wooden aircraft  structure immediately beneath the upper surfaces for signs  of deteriorating adhesives. When checking a glue line (the edge of the glued joint) for  condition, all protective coatings of paint should be removed  by careful scraping. It is important to ensure that the wood is  not damaged during the scraping operation. Scraping should  cease immediately when the wood is revealed in its natural  state and the glue line is clearly discernible. At this point in  the inspection, it is important that the surrounding wood is  dry; otherwise, you will get a false indication of the integrity  of the glue line due to swelling of the wood and subsequent  closing of the joint.  The various cuts of lumber from a tree have tendency to  shrink and warp in the direction(s) indicated in the yellow  area around each cut in  Figure 6-5.  This means that two major structural members joined to each  other by glue are not likely to have identical characteristics.  Over a period of time, differential loads are transmitted  across the glue joint because the two members do not react  identically. This imposes stresses in the glue joint that can  normally be accommodated when the aircraft is new and for  some years afterwards. However, glue tends to deteriorate  with age, and stresses at the glued joints may cause failure of  the joints. This is a fact even when the aircraft is maintained  under ideal conditions. Aircraft that are exposed to large cyclic changes of  temperature and humidity are especially prone to wood  shrinkage that may lead to glue joint deterioration. The  amount of movement of a wooden member due to these  changes varies with the size of each member, the rate of  growth of the tree from which it was cut, and the way the  wood was converted in relation to the grain.  Plain sawed (tangential cut) Quarter sawed (radial cut) Figure 6-5.  Effects of shrinkage on the various shapes during drying  from the green condition. 6-4 Glued Joint Inspection •  Chemical reactions of the glue caused by aging or  moisture, extreme temperatures, or a combination of  these factors. •  Mechanical forces caused mainly by wood shrinkage. •  Development of fungal growths.