Lower cap member Upper cap member Diagonal tube Vertical tube Lower spar cap Upper spar cap Rivets Splice Lower spar web Upper spar web Lower spar cap Upper spar cap Rib attach angle Stiffener Figure 1-30.  A fail-safe spar with a riveted spar web.  Figure 1-27.  A truss wing spar. Figure 1-28.  A plate web wing spar with vertical stiffeners.  other structural members of the wing are placed under load,  most of the resulting stress is passed on to the wing spar. False spars are commonly used in wing design. They are  longitudinal members like spars but do not extend the entire  spanwise length of the wing. Often, they are used as hinge  attach points for control surfaces, such as an aileron spar . Ribs are the structural crosspieces that combine with spars  and stringers to make up the framework of the wing. They  usually extend from the wing leading edge to the rear spar  or to the trailing edge of the wing. The ribs give the wing  its cambered shape and transmit the load from the skin and  stringers to the spars. Similar ribs are also used in ailerons,  elevators, rudders, and stabilizers.  Wing ribs are usually manufactured from either wood or  metal. Aircraft with wood wing spars may have wood or  metal ribs while most aircraft with metal spars have metal  ribs. Wood ribs are usually manufactured from spruce. The  three most common types of wooden ribs are the plywood  web, the lightened plywood web, and the truss types. Of these  three, the truss type is the most efficient because it is strong  and lightweight, but it is also the most complex to construct.  Figure 1-31  shows wood truss web ribs and a lightened  plywood web rib. Wood ribs have a rib cap or cap strip  fastened around the entire perimeter of the rib. It is usually  made of the same material as the rib itself. The rib cap stiffens  and strengthens the rib and provides an attaching surface  for the wing covering. In  Figure 1-31A,  the cross-section  of a wing rib with a truss-type web is illustrated. The dark  rectangular sections are the front and rear wing spars. Note that  to reinforce the truss, gussets are used. In  Figure 1-31B,  a truss  web rib is shown with a continuous gusset. It provides greater  support throughout the entire rib with very little additional  As a rule, a wing has two spars. One spar is usually located  near the front of the wing, and the other about two-thirds of  the distance toward the wing’s trailing edge. Regardless of  type, the spar is the most important part of the wing. When  type of spar breaks, the other section can still carry the load.  This is the fail-safe feature. Figure 1-29.  A sine wave wing spar can be made from aluminum  or composite materials.  Caps Sine wave web Wing Ribs 1-15