Check all metal fittings that are attached to a wood structure  for looseness, corrosion, cracks, or bending. Areas of  particular concern are strut attach fittings, spar butt fittings,  aileron and flap hinges, jury strut fittings, compression struts,  control cable pulley brackets, and landing gear fittings. All  exposed end grain wood, particularly the spar butts, should  be inspected for cracking or checking.  Inspect structural members for compression failures, which  is indicated by rupture across the wood fibers. This is a  serious defect that can be difficult to detect. If a compression  failure is suspected, a flashlight beam shown along the  member and running parallel to the grain, will assist in  revealing it. The surface will appear to have minute ridges  or lines running across the grain. Particular attention is  necessary when inspecting any wooden member that has  been subjected to abnormal bending or compression loads  during a hard landing. If undetected, compression failures  of the spar may result in structural failure of the wing during  flight.  [Figure 6-9] area with a light plastic hammer or screwdriver handle should  produce a sharp solid sound. If the suspected area sounds  hollow and dull, further inspection is warranted. Use a sharp  metal awl or thin-bladed screwdriver to probe the area. The  wood structure should be solid and firm. If the area is soft  and mushy, the wood is rotted and disassembly and repair  of the structure is necessary. The standard for any repair is that it should return the aircraft  or component to its original condition in strength, function,  and aerodynamic shape. It should also be accomplished in  accordance with the manufacturer’s specifications and/or  instructions, or other approved data. When a member has been subjected to an excessive bending  load, the failure appears on the surface that has been  compressed. The surface subject to tension normally shows  no defects. In the case of a member taking an excessive direct  compression load, the failure is apparent on all surfaces. The purpose of repairing all wood structural components  is to obtain a structure as strong as the original. Major  damage probably requires replacement of the entire damaged  assembly, but minor damage can be repaired by removing  or cutting away the damaged members and replacing them  with new sections. This replacement may be accomplished by  gluing, glue and nails, or glue and screw-reinforced splicing. Several forms of wood are commonly used in aircraft.  The front and rear spars should be checked for longitudinal  cracks at the ends of the plywood reinforcement plates  where the lift struts attach.  [Figure 6-8]  Check the ribs on  either side of the strut attach points for cracks where the cap  strips pass over and under the spars, and for missing or loose  rib-to-spar attach nails. All spars, those in the wing(s) and  empennage, should be inspected on the face and top surface  for compression cracks. A borescope can be utilized by  accessing existing inspection holes. Various mechanical methods can be employed to enhance  the visual inspection of wood structures. Tapping the subject  The various species of wood listed in  Figure 6-10  are  acceptable for structural purposes when used for the repair  of aircraft. Spruce is the preferred choice and the standard  by which the other wood is measured.  Figure 6-10  provides  a comparison of other wood that may be suitable for aircraft  repair. It lists the strength and characteristics of the wood  in comparison to spruce. The one item common to all the  species is that the slope of the grain cannot be steeper  than 1:15.  $^{ }$Suitable Wood  Materials Repair of Wood Aircraft Structures •  Solid wood or the adjective “solid” used with such  nouns as “beam” or “spar” refers to a member  consisting of one piece of wood.  •  Laminated wood is an assembly of two or more layers  of wood that have been glued together with the grain  of all layers or laminations approximately parallel. •  Plywood is an assembled product of wood and glue  that is usually made of an odd number of thin plies, or  veneers, with the grain of each layer placed 90° with  the adjacent ply or plies. •  High-density material includes compreg, impreg, or  similar commercially made products, heat-stabilized  wood, or any of the hardwood plywoods commonly  used as bearing or reinforcement plates. Figure 6-9.  Pronounced compression failure in wood beam. Compression failure 6-7