All solid wood and plywood used for the construction and  repair of aircraft should be of the highest quality and grade.  For certificated aircraft, the wood should have traceability to a  source that can provide certification to a military specification  (MIL-SPEC). The term “aircraft quality” or “aircraft grade”  is referred to and specified in some repair documents, but  that grade wood cannot be purchased from a local lumber  company. To purchase the material, contact one of the  specialty aircraft supply companies and request a certification  document with the order. The MIL-SPEC for solid spruce is  MIL-S-6073 and for plywood it is MIL-P-6070B. When possible, fabricated wood components should be  purchased from the aircraft manufacturer, or someone  who may have a Parts Manufacturer Approval (PMA) to  produce replacement parts for the aircraft. With either of  these sources supplying the wood components, the mechanic  can be assured of installing approved material. At the  completion of the repair, as always, it is the responsibility of  the person returning the aircraft to service to determine the  quality of the replacement wood and the airworthiness of the  subsequent repair.  To help determine the suitability of the wood, inspect it  for defects that would make it unsuitable material to repair  or construct an aircraft. The type, location, and amount  or size of the defects grade the wood for possible use. All  woods used for structural repair of aircraft are classified as  softwood. Softwood is typically used for construction and is  graded based on strength, load carrying ability, and safety.  Figure 6-10.  Selection and properties of wood for aircraft repairs. 6-8$^{Species of Wood}$ Strength Properties  (as compared to spruce)  1.15 1.15 1.15 1.15 1.15 1.15 1.15 Maximum  Permissible Grain Deviation (slope of grain)  Excellent for all uses. Considered standard for this table. May be used as substitute for spruce in same sizes or in  slightly reduced sizes if reductions are substantiated.  Difficult to work with hand tools. Some tendency to split and  splinter during fabrication and much greater care in  manufacture is necessary. Large solid pieces should be  avoided due to inspection difficulties. Satisfactory for gluing . Satisfactory characteristics of workability, warping,  and splitting. May be used as direct substitute for spruce in  same sizes if shear does not become critical. Hardness  somewhat less than spruce. Satisfactory for gluing. Less uniform in texture than spruce. May be used as direct  substitute for spruce. Upland growth superior to lowland  growth. Satisfactory for gluing. Excellent working qualities and uniform in properties, but  somewhat low in hardness and shock-resistance.  Cannot be used as substitute for spruce without increase in sizes to compensate for lesser strength. Satisfactory for gluing. May be used as substitute for spruce in same sizes or in  slightly reduced sizes if reductions are substantiated.  Easy to work with hand tools. Gluing is difficult, but satisfactory  joints can be obtained if suitable precautions are taken.  Excellent working qualities. Should not be used as a direct  substitute for spruce without carefully accounting for slightly  reduced strength properties. Somewhat low in shock-resistance. Satisfactory for gluing. Remarks 100% Exceeds spruce Slightly exceeds spruce  except 8% deficient in  shear Slightly exceeds spruce Properties between  85% and 96% those  of spruce Exceeds spruce Slightly less than spruce  except in compression  (crushing) and shear Spruce (Picea)  Sitka (P. sitchensis)  Red (P. rubra)  White (P. glauca) Douglas fir (Pseudotsuga taxifolia)  Noble fir (Abies procera, also known as Abies nobilis) Western hemlock (Tsuga heterophylla) Northern white pine, also known as Eastern white pine (Pinus strobus) Port Orford white cedar (Chamaecyparis  lawsoniana) Yellow poplar  (Liriodendron  tulipifera)  1 2 3 4