A B C D E Figure 1-25.  Typical wooden wing spar cross-sections.  the form of an I-beam. Lamination of solid wood spars is often used to increase strength. Laminated wood can also be  found in box-shaped spars. The spar in  Figure 1-25E  has had  material removed to reduce weight but retains the strength  of a rectangular spar. As can be seen, most wing spars are  basically rectangular in shape with the long dimension of the  cross-section oriented up and down in the wing.  the loads caused by wing bending, with the caps providing a foundation for attaching the skin. Although the spar shapes  in  Figure 1-26  are typical, actual wing spar configurations  assume many forms. For example, the web of a spar may be  a plate or a truss as shown in  Figure 1-27.  It could be built up  from lightweight materials with vertical stiffeners employed  for strength.  [Figure 1-28] Currently, most manufactured aircraft have wing spars  made of solid extruded aluminum or aluminum extrusions  riveted together to form the spar. The increased use of  composites and the combining of materials should make  airmen vigilant for wings spars made from a variety of  materials.  Figure 1-26  shows examples of metal wing spar  cross-sections.  In an I–beam spar, the top and bottom of the I–beam are  called the caps and the vertical section is called the web.  The entire spar can be extruded from one piece of metal  but often it is built up from multiple extrusions or formed  angles. The web forms the principal depth portion of the  spar and the cap strips (extrusions, formed angles, or milled  sections) are attached to it. Together, these members carry  Additionally, fail-safe spar web design exists. Fail-safe  means that should one member of a complex structure fail,  some other part of the structure assumes the load of the failed  member and permits continued operation. A spar with fail- safe construction is shown in  Figure 1-30.  This spar is made  in two sections. The top section consists of a cap riveted to  the upper web plate. The lower section is a single extrusion  consisting of the lower cap and web plate. These two sections  are spliced together to form the spar. If either section of this  It could also have no stiffeners but might contain flanged  holes for reducing weight but maintaining strength. Some  metal and composite wing spars retain the I-beam concept  but use a sine wave web.  [Figure 1-29] 1-14 Figure 1-26.  Examples of metal wing spar shapes. 