performing the repair on an aircraft, regulation requires a  certificated and appropriately rated mechanic or repairman  who has had previous experience in the operation concerned  to supervise that person.  The ability to inspect wood structures and recognize defects  (dry rot, compression failures, etc.) can be learned through  experience and instruction from knowledgeable certificated  mechanics and appropriately qualified technical instructors.  To properly inspect an aircraft constructed or comprised  of wood components, the aircraft must be dry. It should be  placed in a dry, well-ventilated hangar with all inspection  covers, access panels, and removable fairings opened and  removed. This allows interior sections and compartments to  thoroughly dry. Wet, or even damp, wood causes swelling  and makes it difficult to make a proper determination of the  condition of the glue joints.  If there is any doubt that the wood is dry, a moisture meter  should be utilized to verify the percentage of moisture in  the structure. Nondestructive meters are available that check  moisture without making holes in the surface. The ideal range  is 8–12 percent, with any reading over 20 percent providing  an environment for the growth of fungus in the wood. The inspection should begin with an examination of the  external surface of the aircraft. This provides a general  assessment of the overall condition of the wood and structure.  The wings, fuselage, and empennage should be inspected for  undulation, warping, or any other disparity from the original  shape. Where the wings, fuselage, or empennage structure  and skins form stressed structures, no departure from the  original contour or shape is permissible.  [Figure 6-3] Where light structures using single plywood covering are  concerned, some slight sectional undulation or bulging  between panels may be permissible if the wood and glue  are sound. However, where such conditions exist, a careful  check must be made of the attachment of the plywood to its  supporting structure. A typical example of a distorted single  plywood structure is illustrated in  Figure 6-4. The contours and alignment of leading and trailing edges are  of particular importance. A careful check should be made  for any deviation from the original shape. Any distortion  of these light plywood and spruce structures is indicative  of deterioration, and a detailed internal inspection has to be  made for security of these parts to the main wing structure.  If deterioration is found in these components, the main wing  structure may also be affected. Inspection of Wood Structures  External and Internal Inspection Ply skins Ply skins Figure 6-3.  Cross sectional view of a stressed skin structure. Figure 6-4.  A distorted single plywood structure. 6-3