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Aircraft thermal acoustical insulation consists of lightweight fiberglass encased in a thin film bagging material. Practically the entire fuselage is layered with insulation blankets to deaden noise and insulate against heat or cold. Also, heating and air conditioning ducts, in some cases, may be covered with fiberglass blankets. The film bagging material holds the insulation together and is intended to also act as a moisture barrier. Until recently, FAA flammability requirements for thermal acoustical insulation, as prescribed in Federal Aviation Regulation (FAR) 25.853a, consisted of a vertical Bunsen burner test method.

Insulation blankets may be a factor in the prevention of in-flight fires or the mitigation of postcrash fires. In the past, fatal in-flight fires – although relatively rare events – have originated in hidden or inaccessible areas of the aircraft. The preponderance of insulation makes it a likely target for an in-flight ignition fire source and/or as a path for flame propagation and fire growth. With regard to postcrash fire, insulation blankets can provide a barrier against penetration of the fuselage shell by an external fuel fire, commonly referred to fuselage burnthrough. Extending the time for fuselage burnthrough improves survivability by providing additional time for passengers to escape during a postcrash fire.

On August 12, 1999, the FAA issued a proposed airworthiness directive (AD) that would require the replacement of metallized PET, which was employed in insulation blankets on over 700 aircraft. This particular type of film is no longer being installed in commercial transports. It was proposed that the replacement film comply with the new radiant panel test criteria. In addition, the FAA has prepared a Notice of Proposed Rulemaking (NPRM) that would impose new fire test criteria for thermal acoustical insulation based on both the radiant panel test (in-flight fire) and the burnthrough test method (postcrash fire). The final rule was effective September 2, 2003, with a two-year compliance deadline of September 2, 2005 for flame propagation and six-year compliance deadline of September 2, 2009 for burnthrough.

Regulatory Documents