available in a round or flat cross-section. The round cord is  easier to use than the flat lacing, but if installed properly, the  flat lacing results in a smoother finish over the ribs. Sewing of polyester fabric is rare and mostly limited to the  creation of prefitted envelopes used in the envelope method  covering process. When a fabric seam must be made with no  structure underneath it, a sewn seam could be used. Polyester  threads of various specifications are used on polyester fabric.  Different thread is specified for hand sewing versus machine  sewing. For hand sewing, the thread is typically a three-ply,  uncoated polyester thread with a 15-pound tensile strength.  Machine thread is typically four-ply polyester with a 10-  pound tensile strength. Each fabric covering job involves a method of attaching  the fabric to wing and empennage ribs. The original  manufacturer’s method of fastening should be used. In  addition to lacing the fabric to the ribs with approved rib  lacing cord, special clips, screws, and rivets are employed  on some aircraft.  [Figure 3-6]  The first step in using any  of these fasteners is to inspect the holes into which they fit.  Worn holes may have to be enlarged or re-drilled according  to the manufacturer’s instructions. Use of approved fasteners  is mandatory. Use of unapproved fasteners can render the  covering job unairworthy if substituted. Screws and rivets  often incorporate the use of a plastic or aluminum washer.  All fasteners and rib lacing are covered with finishing tape  once installed to provide a smooth finish and airflow. Grommets are used to create reinforced drain holes in the  aircraft fabric. Usually made of aluminum or plastic, they  are glued or doped into place on the fabric surface. Once  secured, a hole is created in the fabric through the center of  the grommet. Often, this is done with a hot soldering pencil  that also heat seals the fabric edge to prevent raveling.  Seaplane grommets have a shield over the drain hole to  prevent splashed water from entering the interior of the  covered structure and to assist in siphoning out any water  from within.  [Figure 3-7]  Drain holes using these grommets  must be made before the grommets are put in place. Note that  some drain holes do not require grommets if they are made  through two layers of fabric. The structure underneath an aircraft covering must be  inspected periodically. To facilitate this in fabric-covered  aircraft, inspection rings are glued or doped to the fabric.  They provide a stable rim around an area of fabric that can be  cut to allow viewing of the structure underneath. The fabric  3-6 Figure 3-6.  Clips, screws, rivets, or lace are used to attach the  fabric to wing and empennage ribs.  Inspection Rings Grommets Special Fabric Fasteners Sewing Thread Rib cap Screws Clips PK screw Rib Fabric Reinforcing tape Washer Martin clip Section of rib Trailing edge Rivets Lace