Weak link Figure 6-12.  Tost hook. Figure 6-11.  Schweizer-type tow hook. Figure 6-9.  Rope strengths. Figure 6-8.  Inspecting the towline. Figure 6-10.  The weak link. Diameter 3/16" 1/4" 5/16" Polyethylene Hollow braid 700 1,200 1,750 Polypropylene Monofilament 800 1,300 1,900 Polypropylene Multifilament 870 1,200 2,050 Nylon 960 1,500 2,400 Dacron 720 1,150 1,750 A knot in the towline reduces its strength by up to 50 percent,  and causes a high spot in the rope that is more susceptible to  wear. Pay particular attention to the ring area to which the  glider attaches because this is also a high-wear area. The safety link is constructed of towline with a towring on  one end and the other end spliced into a loop. The weak link at  the glider attach end of the towline must be 80 to 200 percent  of the maximum certificated operating weight of the glider.  The safety link at the towplane attach end must be of greater  strength than the safety link at the glider attach end of the  towline, but not more than 25 percent greater, nor greater than  200 percent of the maximum certificated weight of the glider.  Towlines and weak links are assembled using a towring that is  appropriate for the operation. Lightweight balls are attached  to the towline to help protect the towline, prevent line rash,  and prevents the line from whipping.  [Figure 6-10] The tow hooks on both the glider and the towplane need to be  inspected. The two most common types of tow hook are an  over-the-top design, such as a Schweizer hook, or a grasping  style, such as a Tost hook. Any tow hook must be freely  operating, and free from damage.  [Figures 6-11  and  6-12] A thorough inspection of the glider should be accomplished  before launch. A preflight checklist for a glider should be  in the GFM/POH. If not, develop a checklist using the  guidelines contained in  Figure 6-13 . Glider Preflight Inspection 6-6