subjected to a flame temperature of not less than 2,000 degrees F for 15 minutes" and therefore don't need to do the specified (and rather involved) Appendix F testing.
If you are just replacing seat covers then it is pretty easy to show that they meet that criteria - they will be several feet away from the firewall. If the fabric is closer to, or in contact with, the firewall, that would raise a requirement for the testing.
If you can't state that, then you just need to comply with the “vertical test” specified.
An alternative to all this is to use the same type of material that manufacturer originally installed (if you have the spec on that - it is often in the aircraft parts manual) or use leather - it complies as it is!
FAR 23 Appendix F specifies the way to do the tests including the conditions and the “vertical test” and says:
“Acceptable test procedure for self-extinguishing materials for showing compliance with §§23.853, 23.855 and 23.1359.
(a) Conditioning. Specimens must be conditioned to 70 degrees F, plus or minus 5 degrees, and at 50 percent plus or minus 5 percent relative humidity until moisture equilibrium is reached or for 24 hours. Only one specimen at a time may be removed from the conditioning environment immediately before subjecting it to the flame.
(b) Specimen configuration. Except as provided for materials used in electrical wire and cable insulation and in small parts, materials must be tested either as a section cut from a fabricated part as installed in the airplane or as a specimen simulating a cut section, such as: a specimen cut from a flat sheet of the material or a model of the fabricated part. The specimen may be cut from any location in a fabricated part; however, fabricated units, such as sandwich panels, may not be separated for a test. The specimen thickness must be no thicker than the minimum thickness to be qualified for use in the airplane, except that: (1) Thick foam parts, such as seat cushions, must be tested in 1/2 inch thickness; (2) when showing compliance with §23.853(d)(3)(v) for materials used in small parts that must be tested, the materials must be tested in no more than 1/8 inch thickness; (3) when showing compliance with §23.1359(c) for materials used in electrical wire and cable insulation, the wire and cable specimens must be the same size as used in the airplane. In the case of fabrics, both the warp and fill direction of the weave must be tested to determine the most critical flammability conditions. When performing the tests prescribed in paragraphs (d) and (e) of this appendix, the specimen must be mounted in a metal frame so that (1) in the vertical tests of paragraph (d) of this appendix, the two long edges and the upper edge are held securely; (2) in the horizontal test of paragraph (e) of this appendix, the two long edges and the edge away from the flame are held securely; (3) the exposed area of the specimen is at least 2 inches wide and 12 inches long, unless the actual size used in the airplane is smaller; and (4) the edge to which the burner flame is applied must not consist of the finished or protected edge of the specimen but must be
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