In general, an aircraft is certified with a certain fabric based  on its wing loading and its never exceed speed (V$_{NE}$). The  higher the wing loading and V$_{NE}$, the stronger the fabric must  be. On aircraft with wing loading of 9 pounds per square foot  and over, or a V$_{NE}$ of 160 miles per hour (mph) or higher,  fabric equaling or exceeding the strength of Grade A cotton is  required. This means the new fabric breaking strength must be  at least 80 pounds and the minimum fabric breaking strength  at which the aircraft becomes unairworthy is 56 pounds. On aircraft with wing loading of 9 pounds per square foot or  less, or a V$_{NE}$ of 160 mph or less, fabric equaling or exceeding  the strength of intermediate grade cotton is required. This  means the new fabric breaking strength must be at least 65  pounds and the minimum fabric breaking strength at which  the aircraft becomes unairworthy is 46 pounds. Lighter weight fabric may be found to have been certified on  gliders or sailplanes and may be used on many uncertificated  aircraft or aircraft in the Light Sport Aircraft (LSA) category.  For aircraft with wing loading less than 8 pounds per square  foot or less, or V$_{NE}$ of 135 mph or less, the fabric is considered  unairworthy when the breaking strength has deteriorated to  below 35 pounds (new minimum strength of 50 pounds).  Figure 3-10  summarizes these parameters. Manufacturer’s instructions should always be consulted  first for fabric strength inspection methodology. These  instructions are approved data and may not require removal  of a test strip to determine airworthiness of the fabric. In some  cases, the manufacturer’s information does not include any  fabric inspection methods. It may refer the IA to AC 43.13- 1, Chapter 2, Fabric Covering, which contains the approved  FAA test strip method for breaking strength.  The test strip method for the breaking strength of aircraft  covering fabrics uses standards published by the American  Society for Testing and Materials (ASTM) for the testing  of various materials. Breaking strength is determined by  cutting a 1¼ inch by 4–6 inch strip of fabric from the aircraft  covering. This sample should be taken from an area that is  exposed to the elements—usually an upper surface. It is also  wise to take the sample from an area that has a dark colored  finish since this has absorbed more of the sun’s UV rays  and degraded faster. All coatings are then removed and the  edges raveled to leave a 1-inch width. One end of the strip is  clamped into a secured clamp and the other end is clamped  such that a suitable container may be suspended from it.  Weight is added to the container until the fabric breaks. The  breaking strength of the fabric is equal to the weight of the  lower clamp, the container, and the weight added to it. If the  breaking strength is still in question, a sample should be sent  to a qualified testing laboratory and breaking strength tests  made in accordance with ASTM publication D5035.  Note that the fabric test strip must have all coatings removed  from it for the test. Soaking and cleaning the test strip in  methyl ethyl ketone (MEK) usually removes all the coatings.  Properly installed and maintained polyester fabric should  give years of service before appreciable fabric strength  degradation occurs. Aircraft owners often prefer not to  have test strips cut out of the fabric, especially when the  aircraft or the fabric covering is relatively new, because  removal of a test strip damages the integrity of an airworthy  component if the fabric passes. The test strip area then must  be repaired, costing time and money. To avoid cutting a  strip out of airworthy fabric, the IA makes a decision based  on knowledge, experience, and available nondestructive  techniques as to whether removal of a test strip is warranted  to ensure that the aircraft can be returned to service. An aircraft made airworthy under an STC is subject to the  instructions for continued airworthiness in that STC. Most  STCs refer to AC 43.13-1 for inspection methodology. Poly- Fiber™ and Ceconite™ re-covering process STCs contain their  own instructions and techniques for determining fabric strength  and airworthiness. Therefore, an aircraft covered under those  STCs may be inspected in accordance with this information.  In most cases, the aircraft can be approved for return to service  without cutting a strip from the fabric covering.  How Fabric Breaking Strength is Determined Figure 3-10.  Aircraft performance affects fabric selection. 3-10 Fabric Performance Criteria IF YOUR PERFORMANCE IS. . . FABRIC STRENGTH MUST BE. . .  Loading Type V$_{NE}$ Speed New Breaking  Strength Minimum Breaking  Strength > 9 lb/sq ft < 9 lb/sq ft < 8 lb/sq ft > 160 mph < 160 mph < 135 mph ≥ Grade A ≥ Intermediate ≥Lightweight > 80 lb > 65 lb > 50 lb > 56 > 46 > 35