Subject: THERMAL/ACOUSTIC INSULATION 

FLAME PROPAGATION TEST METHOD 

DETAILS 

Date: 6/24/05

Initiated By: ANM-115 

AC No: 25.856-1

Change: 

1. PURPOSE. This advisory circular (AC) provides guidance concerning the test method to 

determine the flammability and flame propagation characteristics of thermal/acoustic insulation 

materials. This guidance applies to airplanes required to comply with § 25.856, and part VI of 

appendix F to Title 14 Code of Federal Regulations (14 CFR) part 25. 

2. APPLICABILITY. 

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. RELATED REGULATIONS AND DOCUMENTS. 

a. Title 14 Code of Regulations 25.856 and part VI of appendix F to 14 CFR part 25; 

14 CFR 91.613, 14 CFR 121.312, 14 CFR 125.113, and 14 CFR 135.170. 

b. Advisory Circular 25.856-2, “Installation of Thermal Acoustic Insulation for 

Burnthrough Protection.” An electronic copy of AC 25.856-2 can be downloaded from the 
AC 25.856-1 6/24/05 

Internet at http://www.airweb.faa.gov/rgl. A paper copy may be ordered from the U.S. 

Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business 

Center, 3341 Q 75th Avenue, Landover, MD 20795. 

4. BACKGROUND. 

 a. General. Amendment 25-111 (68 FR 45046, July 31, 2003) added new fire protection 

requirements applicable to thermal/acoustic insulation materials. The term ‘thermal/acoustic’ 

means either thermal or acoustic, or both. The amendment added test requirements in parts VI 

and VII of Appendix F related to flame propagation and burnthrough penetration resistance, 

respectively. Experience has shown the Bunsen burner test that was required prior to the 

adoption of amendment 25-111 did not provide a sufficient measure of thermal/acoustic 

insulation’s resistance to flame propagation. In addition, since most thermal/acoustic insulation 

is installed in parts of the airplane that are not accessible for fire fighting, it is critical that the 

material combinations themselves will not propagate a fire if ignited. 

 b. Construction Details. Thermal/acoustic insulation typically consists of a thin film

moisture barrier encapsulating a batting material. There are also foam insulation materials that 

may or may not use a separate moisture barrier. In any case, in addition to the basic materials 

that make up the insulation, there are frequently detail materials that need to be accounted for. 

These detail materials include thread, tapes, and hook and loop fasteners. With the exception of 

“small parts,” covered in part I of appendix F to part 25, detail materials such as these are 

considered part of the construction of the material and have been shown to have an influence on 

whether the material will propagate a fire. Since these detail features of the construction are not 

uniformly applied and may be used with several different basic materials, fabrication of test 

samples requires special consideration of these detail materials, in addition to that of the basic 

materials.

 c. Test Method. The test described in part VI of appendix F to part 25 uses a radiant panel 

with an ignition source to measure the tendency of thermal/acoustic insulation to propagate a 

fire. The test requires consideration of two parameters as pass/fail criteria: flame propagation 

and flame time after removal of the ignition source. There is an important distinction between 

“flame propagation,” as assessed by this requirement, and “burn length,” as measured in the tests 

described in part I of appendix F. The radiant panel test is intended to measure the actual 

propagation of a flame along the test sample and must be observed as it is happening during a 

test. Burn length, on the other hand, can often be determined by inspecting a test specimen after 

a test is conducted. Burn length includes charring and consumption of the material, regardless of 

whether flaming is evident. Flame propagation is strictly an assessment of the propagation of 

flame from the point of application of the ignition source. The intent of the rule is to limit 

thermal/acoustic insulation to materials that do not propagate a fire. Since a “zero” propagation 

requirement is impractical to implement, reasonable pass/fail criteria were established to meet 

that intent. 

(1) As with any stringent test method, there will be materials that have slightly greater 

propagation characteristics and do not meet the requirement. In some cases this may be due to 

an improperly constructed test sample or variation in the test procedure. To allow for such an 

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6/24/05 AC 25.856-1 

anomaly, the following is an acceptable method of addressing failure of propagation length if 

one of the three samples does not pass. To reach a passing criteria, a minimum of 7 additional 

samples may be tested. None of these additional samples can fail either criterion. In addition, 

the average of all of the samples, including the sample that originally failed, must meet the 

pass/fail criteria. 

(2) For after-flame time only, it is possible that certain materials with good propagation 

protection characteristics can show modest after-flame time, beyond the 3-second limit. There 

are two ways that this can be addressed to determine if the material is acceptable. 

(a) In a manner similar to the procedure identified in paragraph 4c(1) above, test 

an additional 7 samples. If the average of the after-flame times for all 10 samples is 3 seconds or 

less the material can be accepted. Note that, as required in the rule, the propagation distance 

must not exceed 2 inches on any sample. 

(b) In some cases, after-flame can be attributed to the specific 15-second exposure 

time in the test. Since the exposure time was mainly derived from tests on thin film materials, 

extended exposure may be meaningful for other types of materials. If the after-flame time is 

above the limit but the material does not propagate, it is acceptable to apply the burner flame for 

30 seconds. The same pass/fail criteria are applied, and if the material passes with a 30-second 

exposure it can be used. The earlier tests with a 15-second exposure may be disregarded. 

5. TEST SAMPLE CONSTRUCTION. 

a. Except as noted in this paragraph, test samples are constructed as described in part VI of 

appendix F. 

(1) Each combination of affected materials should comply with § 25.856(a). But it 

should be possible to reduce the number of combinations actually tested by establishing the 

critical combinations. For example, different thicknesses of core material, or combinations of 

core materials that are asymmetrical with respect to the outer faces, should not require tests of 

every combination to establish compliance. The same should be true of density of core material. 

The applicant should propose a compliance plan to address such variations. 

(2) A small slit should be cut into film coverings to permit ventilation of gases. The 

slit should be cut toward the end of the sample, away from the pilot burner. Appendix F part VI 

(f)(3) describes adding the approximately 2 inch slit to test samples with film/fiberglass 

composition. The slit is described as “longitudinal.” This can be confusing with respect to the 

orientation of the test sample, or the orientation of the test chamber. The intent was to cut the 

slit from front to rear of the chamber. The slit does not have to be precisely 2 inches. A 1⁄2-inch 

tolerance is acceptable. The slit is for test purposes only and is not a requirement for the 

installed insulation. 

 b. Tape. Tapes are used both during initial production and in the process of making repairs 

or facilitating material replacements on airplanes in service. It is not practical to test each 

possible configuration of tape and film/batting material. To simplify this process, the following 

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AC 25.856-1 6/24/05 

procedure has been developed to show compliance for the use of tape. It is, of course, also 

acceptable to test the tape in its installed configuration, which will qualify that configuration. 

(1) Each type of tape requires qualification on each material on which it is used. It is 

acceptable, however, to use a standard batting material for all tape/film combinations. 

(2) Four strips of 2-inch wide (as supplied by the manufacturer) tape should be placed 

on the test sample. The tape strips should be cut long enough to cover the sample from front to 

back. The first strip of tape is applied at the right side of the sample and each strip of tape 

overlaps the preceding strip by 1⁄2 +/- 1/8 inch in the right to left direction. The tape should be 

arranged so that the centerline of the pilot burner igniter flame contacts the seam of the first 

overlap. Figure 1 shows the general arrangement. It will be necessary to make a template in 

order to know how far from the right edge of the sample to place the first strip of tape. This is 

necessary to ensure that the burner flame contacts properly. 

Adhere four 2-inch strips

of tape from right to left. 

Apply strip 1 first, with a

1⁄2-inch overlap for 

successive strips. 

Tape Orientation 

 

(3) If the tape is not supplied in 2-inch widths, but only in wider sizes, cut the tape 

down to 2 inches. If it is narrower than 2 inches, use the actual width. 

Figure 1 

 c. Hook and Loop. Separate insulation blankets are often connected using hook and loop 

fastening systems. Depending on the installation, hook and loop fasteners could also be used to 

attach the insulation to the airplane structure. As with tape, there are many possible 

combinations and orientations of fastener and film/batting, so that testing each one could be 

impractical. Nonetheless, the influence of the hook and loop on fire safety is potentially 

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6/24/05 AC 25.856-1 

significant and must be addressed in testing. A test procedure has been developed to simplify 

the certification process. This procedure uses test samples that differ from that specified in part 

VI of appendix F. Because the regulation does specify a sample size, it would be acceptable to 

construct samples of that size where the entire surface is covered with hook and loop material as 

applicable. This is considered excessive, however, and the alternative procedure outlined below 

is recommended. 

(1) In order to address the different uses of hook and loop materials, samples are tested 

as mated components. While hook and loop material is not always mated when installed (e.g., 

there may be small segments of unmated material), testing indicates that materials that pass when 

mated are satisfactory. 

(2) Encapsulated material samples may be constructed by folding the material with the 

mated edge around the core material and then stapling the sample along the non-mated edges. 

Figure 2 shows the configuration for mated samples. Note that in this case the sample size and 

orientation relative to the pilot burner are different than for a typical test sample. The 1-inch 

hook and loop strips should be attached within 1/4 inch of the sample edge. 

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AC 25.856-1 6/24/05 

27

4 "/

(108mm)

12"

4 Staples

(305mm)

4 "/

(108mm)

4 "/

(108mm)

12"

(305mm)

Hook

Loop

12"

(305mm)

Back Front

Right

Left

4”

4”

4”

Figure 2 

(3) A small retaining frame must be constructed in order to keep the sample in place. 

The frame is fabricated of mild steel with a thickness of 1/8 inch (3.2 mm). The sample 

retaining frame is shown in Figure 3. Adjusting the height of the sample may also be necessary 

to ensure that the relationship to the pilot burner is maintained to keep the junction of the hook 

and loop exposed to the tip of the burner flame. It is not necessary to adjust the radiant panel 

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6/24/05 AC 25.856-1 

calibration to compensate for the potential change in the location of the exposure upper surface. 

Figure 4 shows the sample with the retaining frame in place. 

Figure 3 

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AC 25.856-1 6/24/05 

Figure 4 

(4) Three tests are conducted with each combination of mated samples using the same

pass/fail criteria as specified in part VI of appendix F. Once this combination of materials (hook 

and loop, whatever material it is attached to, and any other insulation materials contained in the 

sample) have been substantiated, that combination of materials may be mated to any other 

substantiated combination of materials without further test. 

(5) Tests on a 1-inch-thick batting material may be used to substantiate the hook and 

loop on all other thicknesses and densities of that material. This thickness will keep the upper 

surface at the standard height below the radiant panel. Since each core/outer material 

combination will require substantiation in the standard configuration, the hook and loop test may 

be conducted with any acceptable core material. A 1-inch batting is recommended for 

simplicity. Thicker samples are not recommended, because this will increase the heat flux on the 

upper surface. 

d. Insulation that is bonded to a substrate, including ducts, that is not required to comply 

with § 25.856(a) may be tested bonded to a thin (approximately 0.060 inch) aluminum sheet. 

Once qualified in this manner, the insulation may be bonded to any other substrate without 

further test, as long as the substrate itself is not required to meet § 25.856(a). For the purposes of 

compliance with § 25.856(a), insulation adhered to such substrates with double-backed tape may 

be addressed in a similar manner – by testing the sample adhered with the same tape onto an 

aluminum sheet. 

e. Quilted blanket configurations need to be validated and may require separate testing, 

depending on the thread and stitch pattern used. 

6. TEST CONDUCT CONSIDERATIONS. 

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6/24/05 AC 25.856-1 

a. The actual heat flux generated by the radiant panel has a direct influence on test results. 

Experiments suggest that variations in heat flux from the standard can influence both flame time 

(after removal of the pilot burner flame) and propagation length, although not necessarily in the 

same way. A slightly higher heat flux might be critical for one parameter, whereas a slightly 

lower heat flux might be critical for the other parameter. It is essential that heat flux be 

maintained as specified in the regulation. Calibration before and after each day of tests is 

recommended. 

b. In addition to the procedural details described in part VI of appendix F, the test results 

can be significantly influenced by the condition of the chimney in the test chamber. The 

chimney should always be open and clear of obstruction for valid results. 

c. Materials that melt can alter the flame profile of the pilot burner if the material melts to 

the point that the pilot burner flame is directly contacting the backing board. Tests of such 

materials need to be observed closely to determine whether the material is burning or simply 

melting. Material burning beyond the 2-inch limit is considered flame propagation, regardless of 

whether the influence of the flame profile on the backing board is involved. 

d. Occasionally a material may exhibit “flashing” behavior. Flames appear on the test 

sample some distance (i.e., more than 2 inches) away from the zero point. This is also 

considered flame propagation. 

7. APPLICABILITY TO CERTAIN MATERIALS AND INSTALLATIONS. 

a. The regulation applies to thermal/acoustic insulation installed within the fuselage. 

There are four reasons that materials associated with thermal/acoustic insulation would not 

require testing under part VI of appendix F: 

(1) The material is not in the fuselage. 

(2) The material is not thermal/acoustic insulation. 

(3) The material is not so extensive a part of the insulation system that it could have an 

effect on flame propagation. 

(4) The material is installed in a manner that does not permit fire propagation, and this 

can be substantiated by equivalent level of safety finding. 

b. Areas not considered to be in the fuselage include the wing, wing-to-body fairings, 

empennage, and wheel wells. Other areas, including unpressurized portions of the fuselage, are 

covered by the rule. 

c. Certain items are installed in conjunction with thermal/acoustic insulation but are not 

themselves part of the thermal/acoustic insulation. These include small grommets to permit 

fastening of wire bundles or other systems, clamps, small moisture diverters, and small moisture 

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AC 25.856-1 6/24/05 

traps on structural components. Curtains in the cabin, regardless of what insulating properties 

they might possess, are not considered “thermal/acoustic insulation” for the purposes of 

§ 25.856. Similarly, carpet padding is in some cases used to reduce noise. For the purposes of 

§ 25.856, however, carpet padding is not considered thermal/acoustic insulation. It should be 

noted, though, that materials that satisfy § 25.856 are available for carpet padding. 

d. Other materials may be bonded to the insulation but, unlike hook and loop and tapes, are 

not so extensive that they could affect the flame propagation properties of the insulation system. 

These are materials that are installed at limited locations in the airplane, are not contiguous with 

respect to each other, and are generally only a few square inches in area. Examples are moisture 

seals on structural penetrations, small foam standoffs, and ventilation hole reinforcement 

grommets. Additional testing under part VI of appendix F using those materials is not needed to 

demonstrate acceptable flame propagation characteristics. 

e. The following installations are considered to satisfy the requirements of § 25.856 by 

equivalent level of safety under § 21.21(b)(1) because the standards for those installations 

already provide protection against fire propagation. Thus the following installations do not 

require testing under part VI appendix F: 

(1) Insulation installed in an area within the fuselage that is a designated fire zone 

under § 25.1181. 

(2) Insulation within a six-sided box, such as an oven, service cart, or coffee maker, 

where the insulation is between walls, or captured by other structure. Insulation within a 

fuselage door, while typically the same as installed elsewhere in the fuselage, could be 

considered within a six-sided box. 

NOTE: Galley equipment that contains insulation and is ventilated is not 

considered within a six-sided box. However, this insulation still may not have 

to meet the test of part VI of appendix F if the equipment is shown to prevent a 

fire from escaping the equipment. This latter condition will likely need to be 

shown by test. 

(3) Insulation installed inside equipment currently qualified for compliance with 

§ 25.853 on the basis of the guidance in paragraph 623.b. of Advisory Circular 25-17 ( a so–

called “black box”). 

(4) Insulation that is within a class C cargo compartment and fully contained by the 

cargo liner. 

(5) Insulation that meets the requirements of § 25.853(a) and appendix F, part I, 

paragraph (a)(1)(ii) and is installed in the cabin so that it is visible and accessible with a hand 

fire extinguisher. This does not include insulation inside other compartments that involve 

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6/24/05 AC 25.856-1 

opening a door or access panel, or in any location where the source of the fire cannot be 

observed. 

 f. Damping Systems. 

(1) Structural damping materials may be considered thermal/acoustic insulation, 

depending on their specific configuration and use. These materials are often a soft aluminum

sheet bonded to airplane structure. The aluminum sheet itself is not susceptible to flame 

propagation and is not itself “insulation.” Small aluminum sheets bonded directly to the airplane 

skin with a layer of adhesive would not be considered thermal/acoustic insulation. Damping 

materials that include a layer of foam or other material would be treated as thermal/acoustic 

insulation, and thus must be tested for flame propagation characteristics. Since the test method 

in appendix F part VI is not well suited to aluminum, we recommended that those types of 

materials be tested in a manner similar to the testing for mated hook and loop samples, so that 

the interface of the insulating material and substrate is exposed to the tip of the burner flame. 

But the test configuration specified in appendix F can be used. It is acceptable to test these 

materials bonded to an aluminum sheet as per paragraph 5d. They can then be used bonded to 

any other substrate. 

(2) When testing at the interface of the insulating material and substrate, some

adaptation is necessary to precisely size and position the test sample. In order to determine the 

correct width (front to back) for the test sample, place a piece of damping system (material) 

bonded to a 0.060-inch-thick piece of aluminum sheet in the radiant panel test apparatus drawer 

or sliding platform. The sample should be 12 inches long, and be placed so that it touches the 

back wall and right side (corner) of the drawer or platform. With the pilot burner in the down 

position and the burner guide removed, the distance from the bottom of the burner to the front 

edge of the sample should be 1 3/8+/-1/8 inch. Samples can then be sized accordingly. Once 

this is determined, all samples may be cut to that width for a particular test apparatus. This 

measurement and sample size determination are needed for each lab because slight variations 

may exist from one apparatus to another. Figure 5 shows the position of the sample and burner. 

The top of the sample should be at the proper height (7 1⁄2 inches from the radiant panel at the 

“zero” position. Since the igniter angle is 27 degrees, the flame then impinges the interface of 

the sample. 

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AC 25.856-1 6/24/05 

±

±

Figure 5 

g. Ducts: Ducts can fall into two basic categories: ducts that have separate insulation, 

and ducts where the duct wall has some insulating properties. Ducts that do not have separate 

insulation on them (in other words, the duct itself provides whatever insulation capability exists) 

can be treated simply as ducting. Section 25.856 would not apply. Ducting with insulation 

encapsulated in a rigid shell, which can be considered a 'duct within a duct,’ enclosed in a 

manner similar to the insulation in an enclosed box described in paragraph 7e(2), would also not 

be subject to § 25.856. This presumes that the insulation between the inner and outer walls of 

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6/24/05 AC 25.856-1 

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