Technicians inexperienced at rib lacing should seek assistance  to ensure the correct knots are being tied. STC holder videos  are invaluable in this area. They present repeated close-up  visual instruction and guidance to ensure airworthy lacing.  AC 43.13-1, Chapter 2, Fabric Covering, also has in-depth  instructions and diagrams as do some manufacturer’s manuals  and STC’s instructions. When the ribs are laced and the fabric covering completely  attached, the various inspection rings, drain grommets,  reinforcing patches, and finishing tapes are applied. Inspection  rings aid access to critical areas of the structure (pulleys, bell  cranks, drag/anti-drag wires, etc.) once the fabric skin is in  place. They are plastic or aluminum and normally cemented  to the fabric using the approved cement and procedures. The  area inside the ring is left intact. It is removed only when  inspection or maintenance requires access through that ring.  Once removed, preformed inspection panels are used to close  the opening. The rings should be positioned as specified by  the manufacturer. Lacking that information, they should be  positioned as they were on the previous covering fabric.  Additional rings should be installed by the technician if it is  determined a certain area would benefit from access in the  future.  [Figure 3-30] Water from rain and condensation can collect under the fabric  covering and needs a way to escape. Drain grommets serve  this purpose. There are a few different types as described in  the materials section above. All are cemented into position  in accordance with the approved process under which the  work is being performed. Locations for the drain grommets  should be ascertained from manufacturer’s data. If not  specified, AC 43.13-1 has acceptable location information.  Each fabric covering STC may also give recommendations.  Typically, drain grommets are located at the lowest part  of each area of the structure (e.g., bottom of the fuselage,  wings, empennage).  [Figure 3-31]  Each rib bay of the wings  is usually drained with one or two grommets on the bottom  of the trailing edge. Note that drain holes without grommets  are sometimes approved in reinforced fabric.  It is possible that additional inspection rings and drain  grommets have been specified after the manufacture of the  aircraft. Check the Airworthiness Directives (ADs) and  Service Bulletins for the aircraft being re-covered to ensure  required rings and grommets have been installed.  Cable guide openings, strut-attach fitting areas, and similar  features, as well as any protrusions in the fabric covering, are  reinforced with fabric gussets. These are installed as patches  in the desired location. They should be cut to fit exactly  around the feature they reinforce to support the original  opening made in the covering fabric.  [Figure 3-32]  Gussets  made to keep protrusions from coming through the fabric  should overlap the area they protect. Most processes call for  the gusset material to be preshrunk and cemented into place  using the approved covering process cementing procedures.  Finishing tapes are applied to all seams, edges, and over the  ribs once all of the procedures above have been completed.  They are used to protect these areas by providing smooth  aerodynamic resistance to abrasion. The tapes are made from  the same polyester material as the covering fabric. Use of  lighter weight tapes is approved in some STCs. Preshrunk  tapes are preferred because they react to exposure to the  environment in the same way the as the fabric covering.  This minimizes stress on the adhesive joint between the two.  Figure 3-30.  This inspection ring was cemented into place on the  fabric covering. The approved technique specifies the use of a fabric  overlay that is cemented over the ring and to the fabric.  Figure 3-31.  Drain grommets cemented into place on the bottom  side of a control surface.  Rings, Grommets, and Gussets  Finishing Tapes 3-21