Advisory U.S. Department of Transportation CircularFederal Aviation Administration Subject: INSTALLATION OF Date: 7/29/08 AC No: 25.856-2A THERMAL/ACOUSTIC INSULATION FOR Initiated by: ANM-115 Change:
BURNTHROUGH PROTECTION
1. PURPOSE. This AC provides guidance for the test method to determine burnthrough resistance of thermal/acoustic insulation materials installed in transport category airplanes. This guidance applies to airplanes required to comply with § 25.856 and part VII of Appendix F to 14 CFR part 25.
2. APPLICABLITY.
a. The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) transport airplane type certification engineers and their designees.
b. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the relevant regulations. On the other hand, if we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance.
c. This material does not change, create any additional, authorize changes in, or permit deviations from, regulatory requirements.
3. CANCELLATION. This AC cancels AC 25.856-2, Installation of Thermal/Acoustic Insulation For Burnthrough Protection, dated 1/17/06.
4. RELATED REGULATIONS AND DOCUMENTS.
a. 14 CFR 25.856 and part VII of Appendix F to 14 CFR part 25.
b. Advisory Circular (AC) 25.856-1, “Thermal/Acoustic Insulation Flame Propagation Test Method Details.” You can download an electr onic copy of AC 25.856-1 from the Internet at http://rgl.faa.gov. You may order a paper copy from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East business Center, 3341 Q 75th Avenue, Landover, MD 20795.
5. BACKGROUND.
a. Accidents have illustrated the threat that exists due to fuel spillage from damaged aircraft fuel tanks that result in pool fires that penetrate into the cabin. Research has shown that the aluminum skin currently in use offers little opportunity to prevent burnthrough.
Thermal/acoustic insulation having the appropriate fire resistant properties and installed in a controlled manner, however, can delay the onset of fire into the cabin for a sufficient length of time to allow passenger evacuation. The installation methods and materials are critical in deriving the benefit of fire resistant thermal/acoustic insulation.
b. Amendment 25-111 (68 FR 45046, July 31, 2003) introduced a new test method and requirement into part 25 that improves the fire penetration resistance of thermal/acoustic insulation. This AC provides guidance on the installation details and techniques that have been found to be acceptable to realize the full potential of materials having satisfactory fire-resistant properties. Since the primary threat from pool fires is to the lower half of the fuselage, the regulation applies only to the lower half (see paragraph 7e of this AC). The installation criteria specified in this AC are applicable to thermal/acoustic insulation installed in that area; however, use of similar techniques throughout the airplane may be desirable for consistency.
c. The guidance in this AC is mainly directed at insulation systems consisting of a batting encapsulated by a moisture barrier. The insulation is installed by mechanically fixing it to the airframe structure. The guidance in this AC may be inappropriate or not applicable to certain materials and/or installation systems. Where doubt exists as to the relevance of the criteria specified to any particular installation system, perform testing on a burnthrough test rig configured to be representative of a fuselage exposed to a pool fire. See paragraph 8 of this AC for further discussion of this issue.
d. The test method in Appendix F, part VII to part 25 is stringent, and requires that all samples meet the pass/fail criteria specified. As with any stringent test method, there may occasionally be a statistically predictable failure of a material that, as a rule, satisfies the criteria.
In order to address this situation without permanently banning this material from use, the 2
following is an acceptable procedure to address a sample failure. To use the following procedure, only one of the three original sample sets may fail. Note that use of this procedure does require that the test be run longer than 4 minutes, in order that the average burnthrough time be at least 4 minutes.
(1) Test a fourth sample set and average the burnthrough time results of all four tests. If the average exceeds 4 minutes, the material may be used, provided the fourth sample passes the test.
(2) If the fourth sample set should also fail the test, it is acceptable to test an additional two sample sets (for a total of 6) and average the results. If four of the six sample sets pass the test, and the average burnthrough time of all six tests exceeds 4 min, the material can be used.
(3) Consider using materials from more than one lot/batch to make the additional test samples.
e. Section 25.856(b) applies to thermal acoustic insulation that would contribute to post crash fire safety. This regulation has a speci fic provision that excludes from the compliance requirement “thermal/acoustic insulation installations that the FAA finds would not contribute to fire penetration resistance.” The FAA has reviewed numerous installation concepts and established common applications of this provision. These are discussed in paragraph 9 of this AC.
6. EXPLANATION OF TERMS.
a. Burnthrough. The penetration of an external fire into the airplane cabin, typically through the airplane skin, insulation, and sidewall or floor structure. For the purposes of the test, a breach of 0.25” or more in diameter is considered burnthrough.
b. Overlap. The length of insulation material that presents a double thickness of material either against the airplane skin, for the purposes of joining two bags, or abutting airframe structure other than the fuselage skin (see figure 1).
CAPPING STRIP
FUSELAGE SKIN
OVERLAP FRAME
THERMAL ACOUSTIC LINER Figure 1. Overlap
c. Pool Fire. An extensive ground fire originating from fuel spillage from damaged airplane fuel tanks.
d. Thermal/Acoustic Liner. Any materials (for example, a blanket) that are used to thermally or acoustically insulate the interior of the airplane. These materials are typically installed onto the airplane skin or other structure and can form a barrier between the passenger cabin and an external fire. Thermal/acoustic liners consisting of batting encapsulated by a moisture barrier may be known as “bags.”
e. Field Blanket: Thermal/acoustic liner positioned between structural members (frames, for example) and typically fastened on the liner’s periphery.
f. Lower Half: The area of the fuselage below the horizontal line that bisects the cross section of the fuselage. This may be determined using the height of the fuselage as a basis.
7. INSTALLATION OF THERMAL/ACOUSTIC INSULATION.
a. General. As noted previously, the method of installation is very important in realizing the benefits of improved materials. To date, numerous thermal/acoustic insulation materials have been successfully tested. These materials can be classified into three basic categories:
batting systems, barrier systems, and encapsulating systems.
(1) A batting system incorporates a more fire-resistant material to either partially, or fully, replace the industry-standard fiberglass material. In some instances, the system may contain layers of both improved and standard materials.
(2) A barrier system uses a thin, fire-resistant barrier material placed within the standard fiberglass blanket. This barrier can be placed outboard of the insulation batting, inboard of it, or sandwiched between layers.
(3) An encapsulating system incorporates a fire-resistant film cover material, surrounding the batting, that also acts as a fire barrier. A variant of this arrangement could have the fire-resistant film on only one face of the blanket.
Note: Appendix 1 of this AC contains schematic representations of systems that have been tested using actual airplane structure and satisfy the requirement. These schemes can be demonstrated in the test rig, as defined in Appendix F, part VII. Variations from the representations shown in Appendix 1 of this AC that would make the installation more critical (for example, increased fastener pitch) may need to be assessed using a fixture modification such as is shown in paragraph 8 of this AC.
b. Overlap.
(1) Overlap at Frames. Any gaps in the insulation material provide a possible penetration route for fire to enter the cabin. Testing has shown that it is necessary to install insulation bags at frames so they completely cover the frames. Where this is achieved with more than one blanket, a minimum overlap of two inches should be used. Use the total frame height for frames smaller than two inches. Overlaps greater than two inches will provide greater protection times. Ideally, a single insulation blanket would extend over the frame. See figures 2 and 3. In those cases where there is cargo compartment liner meeting the requirements of part III of Appendix F, attached to or abutting the inboard cap of the frame, it is not necessary to overlap the insulation on the cap of the frame. See figure 4.
Figure 2. Method of Overlap at Frame Figure 3. Method of Overlap at Frame 6
Figure 4. Cargo Liner as Part of Barrier
(2) Overlapping of Insulation Blankets. Joints between insulation blankets other than over frames should also be such that overlap of the blankets is provided. Testing has shown that a minimum of 6 inches of overlap is required in order to achieve satisfactory protection. For some materials, it may be possible to demonstrate that less than 6 inches of overlap is acceptable using the test burner. In that case, changing the burner/test stand relationship, so that the burner flame impinges between two of the frames, and on the overlapped area, is an example of an acceptable method to substantiate a lesser overlap (see paragraph 8.c.) Additionally, to reduce potential for fire entry, if the installation considerations permit, blankets should be “shingled” so that the upper blanket overlaps the lower blanket in relation to the fuselage interior. Joints may be secured with a tape or mechanical fasteners. See figure 5. Smaller amounts of overlap may be acceptable if the two blankets are fastened together using a fire-resistant fastening method,
i.e., the melting point of the fastener is at or above the flame temperature. Typical hook and loop fasteners have not proven to be any more effective in delaying burnthrough than with blankets that are not fastened. Joints between blankets within a frame bay are not recommended where the overlap would be at the extreme bottom of the fuselage.
Figure 5. Method of Overlapping
c. Discontinuities.
(1) Terminal blocks, pipe attachments, or any other feature attached to the airplane structure in close proximity to the airplane skin present a possible fire penetration route unless protected. Where practical, the thermal/acoustic liner should be installed so as to minimize the potential for fire penetration. This might be achieved by providing a degree of overlap of the liner, or fabricating the item creating the discontinuity in the liner out of material that is fire resistant.
(2) Certain discontinuities are unavoidable: for example, where essential systems must go from the outboard to the inboard side of the insulation material, and such systems cannot practically be constructed of fire-resistant material themselves. Since the regulation does not mandate installation of thermal/acoustic insulation, such discontinuities cannot be prohibited, although their occurrences should be minimized. Such discontinuities need not be considered in the test samples. The rule, however, does require consideration of the installation design methodology, so discontinuities in the insulation would not be acceptable if they are caused by the installation design methodology.
(3) Alternatives such as intumescent coatings may provide a means to address certain discontinuities, where a change to the installation methodology would be cumbersome. An applicant that proposes to use an intumescent coating in this way should coordinate its proposal with the FAA, since there are at present no standardized methods for testing this approach.
d. Attachment Methods.
(1) General.
(a) Penetration of thermal/acoustic liners should be avoided wherever possible since this results in a possible fire entry point. Attachment methods that do not penetrate the liners, such as over-frame attachments, are preferred, and carry fewer constraints on the type of material they may be constructed from.
(b) Attachment methods that provide good mechanical retention of thermal/acoustic liners are more likely to provide good burnthrough protection, provided they also have the other physical and material properties defined in this AC.
(c) Fasteners that are potentially exposed to the fire and maintain the continuity of the barrier do not require testing if they are of a material whose melting point exceeds the fire temperature. Other such fasteners should be tested. Fasteners that are not exposed to the fire can be made of aluminum or high temperature plastic. Attachments to the structure do not require testing if the attachment to the structure is not critical in maintaining the barrier, e.g., fasteners that maintain a specific shape or form, but whose failure does not introduce a void in the barrier.
(d) The purpose of the tests discussed in paragraphs 8b and 8c of this AC is primarily to ensure the continuity of the barrier, rather than fire resistance of the material system.
Heat flux is not measured in these tests, because the ability of the material to resist heat transfer should have been demonstrated in the basic material test. The installation test shows whether the attachment materials and methods will prevent physical fire penetration.
(2) Through-Frame Attachments. Attachment methods or fasteners that penetrate the insulation bag and frame should be metallic (that is, aluminum, or material with an equivalent melting point). Attachment point spacing (pitch) along the frame should be a maximum of 14 inches. Testing has shown that a pitch of 14 inches will provide acceptable fire penetration resistance. Conversely, testing has also shown that a pitch of less than 14 inches does not provide significantly enhanced protection. See figures 6 and 7.
Figure 6. Frequency of Attachment (Pitch) Figure 7. Through-Frame Fastener 10
Through-frame attachments should be installed as far away from the fuselage skin as practical.
The space between the fuselage skin and the through-frame attachment should be a minimum of 1 inch. Where it is not practical to achieve this amount of space, consideration should be given to alternative attachment methods (for example, over-frame attachments). See figure 8. Note that this AC does not address structural ramifications associated with attachments that penetrate the airframe.
Figure 8. Over frame Attachment
(3) Over-frame Attachments. Fasteners that do not penetrate the frame, but provide attachment for the insulation bags by clipping them over the top of the frame, have been found to be satisfactory in terms of preventing fire penetration at the joints. The design and material of such clips and their pitch should provide good retention of the thermal/acoustic liners. A maximum pitch of 14 inches for over-frame attachments has been demonstrated to be acceptable for compliance. See figures 8 and 9.
Clip Figure 9. Over Frame Blanket Installation
(4) Stringer Attachments. Fasteners that penetrate the thermal/acoustic liner and attach it to stringers should be metallic (that is, aluminum, or material with an equivalent melting point). See figure 10. However, as noted in paragraph 7.(d)(1)(c), fasteners that only maintain the shape or contour of the blanket could be made from any material.
Figure 10. Stringer-Mounted Fastener
e. Lower Half. Section 25.856 requires that thermal/acoustic insulation installed in the lower half of the fuselage comply with the test requirements of part VII of Appendix F for flame penetration resistance. As discussed in the preamble to Amendment 25-111, the requirement applies to thermal/acoustic insulation installed against the fuselage skin, or in another manner that provides burnthrough protection. The regulation does not apply to insulation on ducts installed in the lower half of the fuselage, where the insulation would not contribute to burnthrough protection. The requirement does apply to insulation installed on the floor panels, if there was no insulation installed on the outer fuselage in the lower half. The requirement does not apply to both places when insulation is installed in both places. It is the intent of the regulation that the occupied areas of the airplane have greater fire protection through enhanced burnthrough resistance of the lower half of the fuselage, using installed insulation (see figure
11).
Lower Half Figure 11. Shows two approaches to insulating the lower half of the airplane. On the left, the insulation is installed on the fuselage skin; on the right, the insulation is installed along the floor. Either approach would have to comply with the requirement. But if insulation was installed in both places, it would only have to comply in one place.
Figure 11. Insulation on Lower Half of Airplane
8. OTHER THERMAL/ACOUSTIC INSULATION CONCEPTS. The oil burner test described in part VII of Appendix F is intended to represent the temperature and heat flux approximately equivalent to a post-crash fire. The scale of the test method does not replicate the scale of an actual fire. In addition, the test stand incorporates steel components to facilitate repeated testing, and to eliminate small structural details from the test setup. When materials or installation designs other than those discussed in section 7 are used, the standard test apparatus 12