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9/8/98 AC 43.13-1B 1-27. GENERAL. Inspection of wooden structure includes some methods, equipment, and awareness of failure modes which are unique to wooden aircraft. 1-28. TYPES OF DETERIORATION AND DAMAGE. a. Wood Decay. Wood is an organic product which is subject to attack by fungi. Fungi are plants that grow on and in wood. The moisture content of the wood nominally will have to be 20 percent or greater to sustain fungus growth. The result of this growth is called decay. Decayed wood exhibits softness, swelling if still wet, excessive shrinkage when dry, cracking, and discoloration. Repair or re- place wood if any amount or form of decay is found. b. Splitting. Splits or cracks in wooden members occur along grain lines. When the moisture content of wood is lowered, its di- mensions decrease. The dimensional change is greatest in a tangential direction (across the fi- bers and parallel to the growth rings), some- what less in a radial direction (across the fibers and perpendicular to the growth rings), and is negligible in a longitudinal direction (parallel to the fibers). These dimensional changes can have detrimental effects upon a wood struc- ture, particularly when two parts are bonded together with grains in different directions. This effect can often be seen where a plywood doubler is bonded to a spruce member. As the spruce member dries, it attempts to shrink, but is restrained by the plywood, which shrinks less. The resulting stress in the spruce member exceeds its cross-grain strength, and a split oc- curs. c. Bond Failure. Bond joint failure is generally due to improper fabrication tech- nique or prolonged exposure to moisture in service. Although none of the older adhesives have been specifically found to fail by simple aging, the mechanic is advised to inspect all accessible joints carefully. d. Finish Failure. The finish coat on wood structure (usually varnish) is the last line of defense to prevent water entry into wood and the resulting decay. Finish failure can be the result of prolonged water exposure, wood splitting, ultraviolet light exposure, or surface abrasion. e. Damage. Stress, impact, or mechanical damage to a wood structure is caused by ex- cessive aerodynamic loads or impact loads oc- curring while the aircraft is on the ground. Overtightening of fittings can also cause crushing of the underlying wood member and possible bending of the metal fitting. 1-29. INSPECTION METHODS. When- ever possible, the aircraft should be kept in a dry, well-ventilated hangar, with all inspection covers, access panels, etc., removed for as long as possible before final inspection. The air- craft should be given a preliminary inspection when first removing the inspection covers and access panels and inspected with a moisture meter at this time. If the moisture content is high, the aircraft should be thoroughly dried. If the aircraft is dry, this will facilitate later in- spection, especially when determining the condition of bonded joints. a. Likely locations for wood structure deterioration should be given special attention. Most damage is caused by external influence such as moisture, temperature extremes, or sunlight. Care should be taken to note all pos- sible entry points for moisture, (i.e., cracks or breaks in the finish, fastener holes, inspec- tion/access openings, control system openings, drain holes, and the interfaces of metal fittings Par 1-27 Page 1-11 SECTION 3. INSPECTIONPDF Image | AFS-640
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