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In a majority of survivable accidents accompanied by fire, ignition of the interior of the aircraft is caused by burning jet fuel external to the aircraft. Therefore, the integrity of the aircraft and its ability to provide a barrier against fuel fire penetration is an important factor related to the survival of aircraft occupants. Fuselage burnthrough resistance becomes particularly important when the fuselage remains intact following a crash, which occurs frequently in survivable accidents. The burnthrough resistance may be simplistically viewed as the time interval for a fuel fire to penetrate three fuselage shell members: aluminum skin, thermal acoustical insulation, and the sidewall panel/floor panel combination. Flame penetration may occur in other areas as well, such as windows, air return grills, and seams/joints. The burnthrough resistance of the aluminum skin is well known. It takes only about 30 to 60 seconds for the skin to melt, depending on its thickness. The thermal acoustical insulation becomes the next impediment to burnthrough following the melting of the aluminum skin. In recent years, the FAA conducted several outdoor fuel fire burn tests on surplus fuselages to determine the mechanism and time frame for burnthrough [ DOT/FAA/CT-90/10, July 1994 ]. It was determined that the fiberglass insulation provided an additional 1 to 2 minutes of protection, if it completely covered the fire area and remained in place. Thus, the method of securing the insulation to the fuselage structural members is important. Finally, the sidewall panels/flooring offer the final barrier to fire penetration. Sandwich panels comprised of honeycomb cores and fiberglass facings are effective barriers; however, full-scale fire tests also show that the fire can penetrate into the cabin through air return grills, seams/joints or window reveals. Moreover, some airplanes utilize aluminum sidewall panels that offer minimal burnthrough resistance.

As a result of these initial large scale tests on surplus aircraft, the FAA designed and constructed a reusable test rig for the purposes of evaluating potential improvements that could prevent or delay the burnthrough process in the event of a postcrash fuel fire accident. The construction of a full-scale test rig was the most practical approach to allow for repetitive testing and systematic evaluation of singular components. The full-scale test rig was constructed of steel, and married into a 707 fuselage that had been cut in half and separated. The steel test rig had a 12- by 8-foot section of the skin removed which could be mocked up with aluminum skin, thermal acoustic insulation blankets, and other interior materials such as panels, carpet and cargo liner [ DOT/FAA/AR-98/52, January 1999 ].