How to? ROBERT G. LOCK
Inspect and test aircraft fabric Testing of aircraft fabric dates back to the Grade A TSO C-15 and TSO C-14 days, and the standard for these fabrics must be met by even the most modern synthetic fabric processes on the market today. TSO C-15 (Technical Standard Order) is a woven cotton fabric that must pull test 80 pounds per inch when new. TSO C-14 fabric was an intermediate grade of cotton fabric specifically designed for light low-powered aircraft—this cloth must pull test 65 pounds per inch when new. The deterioration point for these fabrics is 70 percent of original strength new; therefore, TSO C-15 can deteriorate to 56 pounds per inch, and TSO C-14 can deteriorate to 46 pounds per inch. Those standards are in play when testing any type of fabric covering. So how is fabric tested? The most accurate test is a pull test done under controlled conditions in a laboratory. The lab report will give the specific pull test strength when the fabric fails. The most widely used is a field test using a Maule fabric tester. If the fabric is still good, the Maule tester will not punch a hole in the fabric. The older tester was called a Seyboth, and it punctured the fabric to give a reading on its colored bands around the tes12
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Illustration 1
ter. The colors were red, yellow, first, second and third green. The Seyboth tester is most likely not in use anymore as the Maule has replaced it. Aircraft with wing loadings greater than 7 pounds per square foot and VNE speeds (velocity never exceed) greater than 160 mph are mandated to use fabrics that meet the TSO C-15 standard— therefore the deteriorated condition would be 56 pounds per inch. Aircraft with wing loadings less than 7 pounds per square foot and VNE speeds less than 160 mph may use the lighter TSO C-14 fabric as a standard, thus
Illustration 2
the deteriorated strength would be 46 pounds per inch. Therefore, when testing fabric, the VNE and wing loading must be known so the correct deteriorated strength can be determined. Illustration 1 shows an old fabric tensile test from my original Aeronca Champ. The airplane was covered partly with Grade A cotton fabric and partly with Ceconite synthetic fabric. It was painted white with black and red trim, the black and red not good colors for the older fabric processes. In order to have a pull test done, one must cut large holes in the top fabric surfaces so that the pull test sample measures 1 inch by 6 inches in size. Most owners will not stand for a mechanic to cut holes in their fabric, so here is where the Maule fabric tester comes in handy. Note that the tester has a slightly rounded blunt end. That is the part that contacts the fabric surface. As one pushes down against spring pressure, the approximate tensile strength is read on the scale in pounds per inch. When testing any fabric that must meet the TSO C-15 standard, push down until you read 60 pounds, then stop. For an aircraft that must meet the TSOC-14 standard, push down until you read 50 pounds, then stop. Fabric should always be tested on the top surfaces in the darkest color because fabric will deteriorate most when painted a dark color and exposed to UV radiation from the sun. The question that always pops up when discussing fabric testing is when to do it. If I know the airplane, I do not test annually, specifically if the aircraft is covered in a synthetic
process. I do, however, place a strong flashlight inside the fabric to check if any light is transmitted through the finish. If I see light, then I will check fabric tensile strength. If there is no sign of light coming through the finish, I check the finish for cracks, and upon finding none I consider the fabric airworthy. If there are cracks in the finish exposing raw fabric weave, I notify the owner that something must be done to repair those cracks. Sometimes cracks are bad enough that the entire aircraft must be re-covered. Ray Stits did some very interesting experiments; the results can be had by looking in the back of the Poly-Fiber Procedure Manual. If you have never read this data, it’s worth the time to gain knowledge of fabric deterioration. Grade A and Dacron fabric when exposed to ultraviolet light from the sun deteriorates in an alarming rate, thus if cracks expose fabric weave, I consider that as the weakest point of the covering and judge its airworthiness accordingly. The Maule tester is available commercially but is not cheap. However, it is the only method available to field test aircraft fabric covering for airworthiness.
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