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the duct is separate from the outer wall. Separate insulation covering would have to meet § 25.856. However, as noted in paragraph 6d, if the insulation is bonded to the duct, the duct may be represented by thin sheet aluminum.
h. Thermal/acoustic insulation that meets the requirements of § 25.856(a) is considered to satisfy the requirements of § 25.853(a) on those airplanes for which the certification basis would otherwise require testing in accordance with § 25.853. This determination can be made using the criteria of § 25.853, which permits “other approved equivalent methods.”
/s/ Mike Kaszycki, Acting Manager, Transport Airplane Directorate Aircraft Certification Service 13