There are three ways to gain FAA approval to re-cover an  aircraft with materials and processes other than those with  which it was originally certificated. One is to do the work in  accordance with an approved supplemental type certificate  (STC). The STC must specify that it is for the particular  aircraft model in question. It states in detail exactly what  alternate materials must be used and what procedure(s) must  be followed. Deviation from the STC data in any way renders  the aircraft unairworthy. The holder of the STC typically sells  the materials and the use of the STC to the person wishing  to re-cover the aircraft.  The second way to gain approval to re-cover an aircraft with  different materials and processes is with a field approval. A  field approval is a one-time approval issued by the FAA Flight  Standards District Office (FSDO) permitting the materials  and procedures requested to replace those of the original  manufacturer. A field approval request is made on FAA Form  337. A thorough description of the materials and processes  must be submitted with proof that, when the alteration is  completed, the aircraft meets or exceeds the performance  parameters set forth by the original type certificate.  The third way is for a manufacturer to secure approval  through the Type Certificate Data Sheet (TCDS) for a new  process. For example, Piper Aircraft Co. originally covered  their PA-18s in cotton. Later, they secured approval to  recover their aircraft with Dacron fabric. Recovering an older  PA-18 with Dacron in accordance with the TCDS would be  a major repair, but not an alteration as the TCDS holder has  current approval for the fabric.  Advisory Circular (AC) 43.13-1, Acceptable Methods,  Techniques, and Practices—Aircraft Inspection and Repair,  contains acceptable practices for covering aircraft with fabric.  It is a valuable source of general and specific information on  fabric and fabric repair that can be used on Form 337 to justify  procedures requested for a field approval. Submitting an FAA  Form 337 does not guarantee a requested field approval. The  FSDO inspector considers all aspects of the procedures and  their effect(s) on the aircraft for which the request is being  filed. Additional data may be required for approval.  Title 14 of the Code of Federal Regulations (14 CFR) part  43, Appendix A, states which maintenance actions are  considered major repairs and which actions are considered  major alterations. Fabric re-covering is considered a major  repair and FAA Form 337 is executed whenever an aircraft is  re-covered with fabric. Appendix A also states that changing  parts of an aircraft wing, tail surface, or fuselage when not  listed in the aircraft specifications issued by the FAA is a  major alteration. This means that replacing cotton fabric with  polyester fabric is a major alteration. A properly executed  FAA Form 337 also needs to be approved in order for this  alteration to be legal.  FAA Form 337, which satisfies the documentation requirements  for major fabric repairs and alterations, requires participation of  an FAA-certificated Airframe and Powerplant (A&P) mechanic  with an Inspection Authorization (IA) in the re-covering  process. Often the work involved in re-covering a fabric aircraft  is performed by someone else, but under the supervision of the  IA (IA certification requires A&P certification). This typically  means the IA inspects the aircraft structure and the re-cover job  at various stages to be sure STC or field approval specifications  are being followed. The signatures of the IA and the FSDO  inspector are required on the approved FAA Form 337. The  aircraft logbook also must be signed by the FAA-certificated  A&P mechanic. It is important to contact the local FSDO before  making any major repair or alteration.  There are a variety of approved materials used in aircraft  fabric covering and repair processes. In order for the items  to legally be used, the FAA must approve the fabric, tapes,  threads, cords, glues, dopes, sealants, coatings, thinners,  additives, fungicides, rejuvenators, and paints for the  manufacturer, the holder of an STC, or a field approval. A Technical Standard Order (TSO) is a minimum performance  standard issued by the FAA for specified materials, parts,  processes, and appliances used on civil aircraft. For example,  TSO-C15d, Aircraft Fabric, Grade A, prescribes the minimum  performance standards that approved aircraft fabric must  meet. Fabric that meets or exceeds the TSO can be used as a  covering. Fabric approved to replace Grade-A cotton, such as  polyester, must meet the same criteria. TSO-C15d also refers  to another document, Society of Automotive Engineers (SAE)  Aerospace Material Specification (AMS) 3806D, which details  properties a fabric must contain to be an approved fabric for  airplane cloth. Lighter weight fabrics typically adhere to the  specifications in TSO-C14b, which refers to SAE AMS 3804C.  When a company is approved to manufacture or sell an  approved aviation fabric, it applies for and receives a Parts  Manufacturing Approval (PMA). Currently, only a few  approved fabrics are used for aircraft coverings, such as  the polyester fabrics Ceconite™, Stits/Polyfiber™, and  Superflite™. These fabrics and some of their characteristics  are shown in  Figure 3-4 . The holders of the PMA for these  fabrics have also developed and gained approval for the  various tapes, chords, threads, and liquids that are used  in the covering process. These approved materials, along  with the procedures for using them, constitute the STCs for  each particular fabric covering process. Only the approved  Fabric  Approved Materials  3-4