Figure 6-20B  illustrates a compression rib of the type that is  a standard rib with rectangle compression members added to  one side and a plywood web to the other side. The method  used in this repair is essentially the same as in  Figure 6-20A ,  except that the plywood reinforcement plate, shown in  Figure 6-20B-B , is continued the full distance between the  spars. Figure 6-20C  illustrates a compression rib of the I type with  a rectangular vertical member on each side of the web. The  method of repair is essentially the same as in  Figure 6-20A ,  except the plywood reinforcement plates on each side, shown  in  Figure 6-20C-C , are continued the full distance between  the spars. Wood wing spars are fabricated in various designs using solid  wood, plywood, or a combination of the two.  [Figure 6-21] When a spar is damaged, the method of repair must conform  to the manufacturer’s instructions and recommendations. In  the absence of manufacturer’s instructions, contact the FAA  for advice and approval before making repairs to the spar and  following recommendations in AC 43.13-1. If instructions  are not available for a specific type of repair, it is highly  recommended that you request appropriate engineering  assistance to evaluate and provide guidance for the  intended repair.  Shown in  Figure 6-22  is a recommended method to repair  either a solid or laminated rectangle spar. The slope of the  scarf in any stressed part, such as a spar, should not be steeper  than 15 to 1. Compression ribs are of many different designs, and the  proper method of repairing any part of this type of rib  is specified by the manufacturer. All repairs should be  performed using recommended or approved practices,  materials and adhesives.  Figure 6-20A  illustrates the repair of a compression rib of  the I section type (i.e., wide, shallow cap strips, and a center  plywood web with a rectangular compression member on  each side of the web). The rib damage suggests that the upper  and lower cap strips, the web member, and the compression  members are cracked completely through. To facilitate  this repair, cut the compression members as shown in  Figure 6-20D  and repair as recommended using replacement  sections to the rear spar. Cut the damaged cap strips and  repair as shown in  Figure 6-20 , replacing the aft section of  the cap strips. Plywood side plates are then bonded on each  side diagonally to reinforce the damaged web as shown in  Figure 6-20, A-A . Unless otherwise specified by the aircraft manufacturer, a  damaged spar may be spliced at almost any point except at  wing attachment fittings, landing gear fittings, engine mount  fittings, or lift-and-interplane strut fittings. These fittings may  not overlap any part of the splice. The reinforcement plates  of the splice should not interfere with the proper attachment  or alignment of the fittings. Taper reinforcement plates on  the ends at a 5:1 slope  [Figure 6-23] . The use of a scarf joint to repair a spar or any other component  of an aircraft is dependent on the accessibility to the damaged  section. It may not be possible to utilize a scarf repair where  recommended, so the component may have to be replaced.  A scarf must be precisely cut on both adjoining pieces to  ensure an even thin glue line; otherwise, the joint may not  achieve full strength. The primary difficulty encountered in  making this type of joint is obtaining the same bevel on each  piece.  [Figure 6-24] 6-15 Figure 6-19. Rib trailing edge repair. Figure 6-18.  Relationship of scarf slope to grain slope. Wing Spar Repairs  Incorrect A Incorrect B Correct C Top Plywood, nail, and glue Spruce block Damaged area Damaged area