The procedures in the Poly-Fiber™ and Ceconite™ STCs  outlined in the following paragraphs are useful when inspecting  any fabric covered aircraft as they add to the information  gathered by the IA to determine the condition of the fabric.  However, following these procedures alone on aircraft not re- covered under these STCs does not make the aircraft airworthy.  The IA must add his or her own knowledge, experience, and  judgment to make a final determination of the strength of the  fabric and whether it is airworthy. Exposure to UV radiation appreciably reduces the strength of  polyester fabric and forms the basis of the Poly-Fiber™ and  Ceconite™ fabric evaluation process. All approved covering  systems utilize fill coats applied to the fabric to protect it  from UV. If installed according to the STC, these coatings  should be sufficient to protect the fabric from the sun and  should last indefinitely. Therefore, most of the evaluation  of the strength of the fabric is actually an evaluation of the  condition of its protective coating(s). Upon a close visual inspection, the fabric coating(s) should  be consistent, contain no cracks, and be flexible, not brittle.  Pushing hard against the fabric with a knuckle should not  damage the coating(s). It is recommended the inspector  check in several areas, especially those most exposed to the  sun. Coatings that pass this test can move to a simple test  that determines whether or not UV light is passing through  the coatings. This test is based on the assumption that if visible light  passes through the fabric coatings, then UV light can also. To  verify whether or not visible light passes through the fabric  coating, remove an inspection panel from the wing, fuselage,  or empennage. Have someone hold an illuminated 60-watt  lamp one foot away from the exterior of the fabric. No light  should be visible through the fabric. If no light is visible, the  fabric has not been weakened by UV rays and can be assumed  to be airworthy. There is no need to perform the fabric strip  strength test. If light is visible through the coatings, further  investigation is required. Mechanical devices used to test fabric by pressing against  or piercing the finished fabric are not FAA approved and  are used at the discretion of the FAA-certificated mechanic  to form an opinion on the general fabric condition. Punch  test accuracy depends on the individual device calibration,  total coating thickness, brittleness, and types of coatings  and fabric. If the fabric tests in the lower breaking strength  range with the mechanical punch tester or if the overall  fabric cover conditions are poor, then more accurate field  tests may be made. Seyboth and Maule fabric strength testers designed for  cotton- and linen-covered aircraft, not to be used on modern  Dacron fabrics. Mechanical devices, combined with other  information and experience, help the FAA-certificated  mechanic judge the strength of the fabric.  [Figure 3-11] A second type of punch tester, the Seyboth, is not as popular  as the Maule because it punctures a small hole in the fabric  when the mechanic pushes the shoulder of the testing unit  against the fabric. A pin with a color-coded calibrated scale  protrudes from the top of the tester and the mechanic reads  this scale to determine fabric strength. Since this device  requires a repair regardless of the strength of the fabric  indicated, it is not widely used.  The Maule punch tester, a spring-loaded device with its  scale calibrated in breaking strength, tests fabric strength  by pressing against it while the fabric is still on the aircraft.  It roughly equates strength in pounds per square inch (psi)  of resistance to breaking strength. The tester is pushed  squarely against the fabric until the scale reads the amount  of maximum allowable degradation. If the tester does  not puncture the fabric, it may be considered airworthy.  Punctures near the breaking strength should be followed  with further testing, specifically the strip breaking strength  test described above. Usually, a puncture indicates the fabric  is in need of replacement. The test should be performed on exposed fabric where there  is a crack or chip in the coatings. If there is no crack or chip,  coatings should be removed to expose the fabric wherever  the test is to be done.  80 70 60 50 40 30 75 65 55 45 35 25 M A U L E Maule tester Red Yellow Green Calibrated scale Seyboth or punch tester Fabric Fabric Fabric Testing Devices Figure 3-11.  Seyboth and Maule fabric strength testers. 3-11