Page last updated 06/08/2006
From the beginning of flight to sometime around the early 1950?s the only fabrics available for covering airplanes were natural fiber in origin. Irish linen made from flax or Grade A cotton became the favorites. These fabrics were pulled tight over the aircraft, sewn and stitched to the airframe, they then had various layers of Nitrate or Butyrate dopes applied which shrunk the fabric still further to make a tight finish, finishing paints were naturally applied last. Lines and Cotton fabrics came in different weights which would be chosen dependent on the aircraft VNE wing loading and horsepower. From the 1950?s on man made fabrics became available, these included Polyesters and Fiberglass. Polyester manufactured by DuPont is known as Dacron, this is the most popular product on the market today. The type of Polyester used on microlight aircraft can vary from the basic woven cloth, heat shrunk and doped, in the same manner as Linen or cotton, to Dacron sail cloth which is resin stabilized and pigmented to give a finished product with out any additional work from the builder. Once again polyester comes in various weights to suite the need. Coloured Dacron sail cloth is mostly available in 3.8oz or 4.1oz.
Organic fibers have a finite life due to mildew, fungus and greater susceptibility to ultraviolet degradation, the life span of organic fabrics would normally be between 10 and 20 years. By contrast the life span of polyester fabrics properly applied, doped and painted is almost infinite. It is important to note that there are a number of approved systems for applying polyester to an aircraft. The common systems are Ceconite, Stits Fiber, Polly Fiber and Super Flight; they all have STC (Supplementary Type Certificate) approval for application to various aircraft. STC is a US FAA system of approval for modifications or additions to aircraft. Microlights are not required to follow the STC protocols but it would be unwise to follow an unapproved process. Superflight System VI is a UV stabilized two pot urethane process which can give a high gloss finish to a fabric aircraft. The key to these systems is the effort required to build a suitable UV barrier below the finish coats, this is important as UV is the only possible cause of polyester failure, assuming it?s attached to the aircraft properly.
Dacron sail cloth is in a different category, it wont rot although mildew can grow on it, but as it does not have layers of UV block applied it is very susceptible to UV degradation. It is easy to see the ravages of sunlight on sail cloth as it very quickly fades and in doing so loses its strength. It was once thought that white sail cloth had the best longevity but tests in NZ have shown that the coloured cloth lasts longer, this is probably due to the pigments in coloured cloth protecting the inner fibers where as white cloth has no pigments and allows the UV to penetrate deeper and do more damage. So what is the life span of Dacron sail cloth? This depends heavily on the use the aircraft gets and how it is hangered and covered, but in New Zealand you can expect to have to replace your sails at between 10 to 20 years maybe less for a well used club aircraft.
With such a wide range of life spans it is important to have a testing regime which suites the various fabrics and systems. The FAA AC 43.13-1b states that fabric on any aircraft should be within 70% of original un-doped strength but also sets minimum tensile strengths for airspeeds and wing loadings. I.e. The fabric on an aircraft with a wing loading less than 9lb sq/ft and a VNE less than 160mph should have a tensile strength not less than 46lb per inch width, for an aircraft above these limits the tensile strength should be 56lb, for gliders with a wing loading of 8lb sq/ft and a VNE les than 135mph the tensile strength should be 35lb. The approved test method is a little more involved. A section of cloth is to be removed from the aircraft, trimmed to a 1 inch wide strip and the paint and dope removed. This is then clamped evenly at both ends and hung with a bucket of sand underneath. Sand is added until the fabric tears, the sand is then weighed, this gives the exact value of tensile strength, if the fabric passes this test the area where the fabric was removed is then repaired in an approved manner. As this is a destructive process various methods were developed to try and analyze the fabric in a less destructive way. One method was the Seyboth cone, a weighted cone which was placed point first on the fabric, if it penetrated then the full test was performed, another more popular tool was the ?Maul? tester, a spring gauge which measures pressure applied to the fabric via a small point. By knowing the minimum pressure before penetration it is possible to test fabric without damage, once again if the tester penetrated the cloth then the full test should be preformed. AC 43.13-1b states ?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 mechanic to base an opinion on the general fabric condition. Punch test accuracy will depend on the individual device calibration, total coating thickness, brittleness, and types of coatings and fabric.?
None the less a variant of the Maul tester is ideal for Dacron sail cloth, it works in the same way, a 3mm point with a flat end is fitted to the end of spring gauge. (A belt tension gauge serves the purpose just right.) As Dacron sail cloth does not have coatings applied to it the results will be very consistent with the quality of the fabric being tested. RAANZ has done considerable research in this area in the past and has come up with pressure values based on the type of craft we fly. For double surface aircraft like the Bantam the pressure should be 10lb and for single surface aircraft a figure of 6lb is acceptable.
In conclusion if the fabric on a doped or urethane finished aircraft looks to be in good order and was applied in an approved manner there should be no need to subject the fabric to a punch type test. If the fabric looks old and scruffy then some form of test might be appropriate, the best course of action here might be to contact a local expert, LAME or the current Tech officer before proceeding with any destructive tests, but keep in mind the safety of the owner or pilot of the aircraft in question. For Dacron sail cloth there is no question, the fabric should be checked regularly, at least annually, with a Maul type tester. RAANZ will soon have a supply of testers in stock; they will be available via the administrator. Fiberglass cloth has not been mentioned here as its life is longer than most airframes and there is no appropriate test.
Some notes on using a Maul type tester. The tester should be held perpendicular to the surface being tested. It should be depressed slowly while listening for any for cracking sounds in the fabric. All colours and surfaces should be tested. The areas under greatest tension should be tested, tap the sail to find the highest pitch.
When inspecting a powered parachute have the canopy taken to a parachute packer for inspection, enter the result in the aircraft log and on the inspection form. Your RAANZ fabric tester is of no use here.
The URLs below contain more detailed information for those who are interested in further reading.
USA FAA AC43.13-1b Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair
The following links will require an internet connection.
AWB 51-1 Issue 1, 25 December 2001 Aircraft Fabric Coverings
Aircraft fabric testing, ultralight aircraft sail fabric testing, how to test fabric on ultralights light sport aircraft.
Superflite System VI
Ceconite and Randolph Covering System
Stits Poly-Fiber Fabric Covering System