Aviation Investigation Report A98H0003

1.14.12  Fire Propagating Materials

  1. - Insulation Blankets – General
  2. - Past Known Occurrences
  3. - Contamination Effects on Insulation Blanket Cover Material  Insulation Blankets – General

Thermal acoustic insulation materials are used extensively throughout the aircraft fuselage to maintain comfortable cabin temperatures, and to reduce the noise entering the passenger cabin and cockpit (see Figure 4). While material, such as fluoropolymer composite or polyethylene foams have been used for this purpose, the most popular choice is the insulation blanket. These insulation blankets are typically installed immediately adjacent to the inside of the fuselage skin, over the frames and around the outside of air conditioning ducts.

Insulation blanket construction consists of a batt of fibreglass insulating material encapsulated by a protective cover in the form of a thin moisture barrier film. This protective cover is a composite construction in which a thin web-like polyester or nylon scrim can be glued to the film material for the purpose of producing a tear-stop. Splicing tape may also be used to seal several insulation blankets into a single unit. Thermal acoustic insulation materials must comply with flammability requirements described in FAR 25.853, Appendix F.

Factors that are considered when selecting the cover material for the blankets include durability, fire resistance, weight, impermeability, and ease of fabrication. Two materials that are widely used in the aviation industry and that were used in the occurrence aircraft are polyethylene terephthalate (PET) and PVF. PET material is commonly known as Mylar®,[69] and PVF material is commonly known as Tedlar®.[70] Both materials could be either metallized or non-metallized, and both were approved for use based on the applicable FAA certification tests at the time.

The flammability test used to certify MPET-covered insulation blankets was the vertical Bunsen burner test (see Section This test involved suspending a strip of insulation material vertically over a Bunsen burner, applying flame for 12 seconds, and then removing the flame. To pass the test, a minimum of three specimens of insulation blanket material must self-extinguish within an average flame time of 15 seconds after the flame is removed. Also, the average burn length must not exceed 20 cm (8 inches), and drippings from the insulation blanket material must not flame for longer than an average of 5 seconds. MPET-covered insulation blankets met these requirements: when exposed to the Bunsen burner, it immediately shrivelled up and shrank away from the burner and did not ignite.

Douglas used MPET-covered insulation blankets in various models of production aircraft between 1981 and 1994. The use of MPET-covered insulation blankets was superseded by non-metallized PET-covered insulation blankets.

In the occurrence aircraft, which was built in 1991, MPET-covered insulation blankets were used to insulate the fuselage. They were also used to insulate some of the air conditioning ducts. Most of the air conditioning ducts were insulated with metallized polyvinyl fluoride (MPVF)–covered insulation blankets (see Figure 4).  Past Known Occurrences

Between November 1993 and March 1999, seven known occurrences took place in which either MPET- or MPVF-covered insulation blankets had been ignited and propagated flame. These occurrences involved one MD-87, one MD-82, two B737-300s in 1994 and 1995, and three MD-11s in 1995. (STI1-96)

The ignition source for each fire was relatively small, including wire arcing, hot metal shavings, and a ruptured light ballast case. In all but one instance, the fires occurred when the aircraft was on the ground. In this one instance, the time the fire occurred could not be determined, as the damage was only discovered during subsequent maintenance.

The Civil Aviation Administration of China (CAAC) investigated three of the above occurrences: the two Boeing 737 aircraft that had PET and one MD-11 aircraft that had MPET. According to the documentation available, the CAAC conducted testing on the PET-covered insulation blanket material (Boeing material specification BMS8-142). It was found that once ignited, the material would be completely consumed by fire. In a report dated 24 May 1996, which was forwarded to the FAA, the CAAC recommended that the manufacturer be advised that "the insulation blanket installed in the Boeing 737-300, [and] MD-11 airplanes is fire flammable. They should make a prompt and positive response."

In a response to the CAAC report dated 24 July 1996, the FAA stated that they intended to investigate the behaviour of insulation blanket materials under larger scale conditions. The FAA also stated that, while the tests conducted by the CAAC on the PET were illustrative, the type of CAAC testing conducted (igniting at the sewn edge of the sample material) was not required for certification.

On 9 August 1996, Douglas released an AOL to operators of several of its aircraft types concerning insulation blankets. The AOL contained the following information:

As a result of recent MD-80 and MD-11 ground fire incidents involving insulation blankets covered with metallized Mylar material, Douglas has examined its methods for flammability testing of insulation blankets. We have concluded that an expanded set of test conditions, which includes additional ignition conditions beyond those previously required, better determines blanket flammability characteristics. All insulation blanket materials delivered on Douglas manufactured aircraft have met the applicable requirements for FAA certification. Douglas recommends that operators discontinue use of the reference (D) metallized Mylar blanket covering material and reference (E) tapes. Douglas also recommends that Douglas' expanded test criteria, which is published in the enclosed reference (C) DMS 2446, be applied when operators are replacing blankets in aircraft in-service....

Douglas has made the FAA and industry aware of our conclusions relative to flammability testing and is participating in an FAA/Industry Flammability Working Group that is addressing testing methods and the flammability of materials such as those used for insulation blankets. The working group will perform flammability tests of blanket covering materials from known suppliers, testing specimens of different sizes in different test setups. From the data derived from the working group tests and from tests that Douglas will continue to conduct, Douglas intends to develop an even more rigorous set of flammability test requirements.

DMS 2446 was dated 5 August 1996 and introduced a particular flammability test that was to be used by all McDonnell Douglas suppliers of insulation blanket assemblies. The test, developed by aircraft manufacturers, involved exposing a sample of the insulation blanket assembly to ignited cotton swabs saturated with isopropyl alcohol (the cotton swab test). McDonnell Douglas had found that when tested by this method, MPET-covered insulation blankets ignited and continued to burn.

The AOL went on to explain that McDonnell Douglas was currently installing PET-covered blankets in production aircraft, and that they were seeking to identify improved materials that would meet a more rigorous set of flammability test conditions, while at the same time meeting other desirable characteristics. Ultimately, McDonnell Douglas issued an SB, dated 31 October 1997, that recommended that MD-11 operators remove and replace MPET-covered insulation blankets with MPVF-covered insulation blankets. The SB also declared that MPVF-covered insulation blankets would be used in production aircraft.

In November 1989, the FAA, along with other regulatory authorities and various industry representatives, formed the International Aircraft Materials Fire Test Working Group. In 1996 to 1997, this working group conducted testing to evaluate the performance of various insulation blanket cover materials against both the Bunsen burner certification test and the cotton swab industry tests. This resulted in a document disseminated by the US Department of Transportation, dated September 1997 and entitled Evaluation of Fire Test Methods for Aircraft Thermal Acoustical Insulation (DOT/FAA/AR-97/58). The following excerpts are from this document:

This report presents the results of laboratory round robin flammability testing performed on thermal acoustical insulation blankets and the films used as insulation coverings. This work was requested by the aircraft industry as a result of actual incidents involving flame propagation on the thermal acoustical blankets. Vertical flammability testing was performed as specified in Federal Aviation Regulation (FAR) 25.853, Appendix F. In addition, a cotton swab test developed by the aircraft manufacturers was also evaluated. These cotton swab tests were performed by placing ignited alcohol-saturated cotton swabs on a test-sized blanket and measuring the longest burn length. Test results indicated that the cotton swab tests produced consistent test results, whereas the vertical flammability tests did not. This was especially apparent with one particular film covering which passed the vertical test according to 50% of the participating laboratories while this same film during the cotton swab tests was reported to have been consumed by all but one laboratory which reported that 75% of the sample was destroyed....

Metallized poly (vinyl fluoride) (PVF) and metallized and nonmetallized polyester poly (ethylene terephthalate) (PET) are currently the most widely used films in the aircraft industry....

In air, PET burns with a smoky flame accompanied by melting, dripping, and little char formation. Therefore, fire-retardant treatments are necessary. The fire-retardent treated grades are generally prepared by incorporating halogen or nonhalogen containing materials as part of the polymer molecules or as additives. Metal oxide synergists such as antimony oxide are frequently included. Although fire retardent PET films are resistant to small ignition sources in low heat flux environments, they can burn readily in fully developed fires....

In air, biaxially oriented PVF film (Tedlar) has burn characteristics similar to PET film....

The front face of the metallized PET blanket sample was totally consumed when subjected to the cotton swab test. This was reported by all but one lab, which reported that 75% of the front face was consumed. This is in sharp contrast to the vertical flammability test results which indicated that the metallized PET/fiberglass samples (compressed and noncompressed) passed most of the time. Hence, the cotton swab test proved itself to be a more reproducible test than the vertical flammability test for this particular film/fiberglass assembly....

The grade of metallized PET film evaluated in this round robin is flammable and possibly could propagate a fire in a realistic situation.

The Evaluation of Fire Test Methods for Aircraft Thermal Acoustical Insulation document described five incidents (as included in the seven occurrences noted above) that occurred between 1993 and 1995 and involved flame propagation on insulation blankets.

Subsequent to the SR 111 accident, in a release dated 14 October 1998, the FAA administrator stated that the FAA would develop a new test specification for aircraft insulation materials and would require that existing materials be replaced with insulation that would pass the new test. On 20 September 2000, the FAA issued an NPRM that advocated upgraded flammability standards for all thermal acoustic insulation materials.

By the end of 1997, McDonnell Douglas had discontinued the use of both MPET- and PET-covered insulation blankets in production aircraft. However, use of MPET-covered insulation blankets continued until 2000, when the FAA issued ADs requiring the removal of such blankets from existing aircraft. The FAA also issued an NPRM proposing new flammability standards for thermal acoustic insulation materials.  Contamination Effects on Insulation Blanket Cover Material

Testing carried out on behalf of the TSB showed that several materials found in the fire-damaged area of SR 111 are flammable even before they are exposed to their intended operating environment; that is, they are flammable in an uncontaminated state. Examples of such flammable materials include insulation blanket cover materials, splicing tapes, polyethylene foam, and silicone elastomeric end caps.

Little industry guidance is available to quantify the effects of contamination. According to documentation from various sources,[71] the flammability characteristics of materials can degrade while in service; that is, when they are exposed to contaminants such as dust, lint, adhesives, grease, oil, or corrosion inhibitors.[72] No corroborating test results were available to support this information. In some flammability testing conducted by the TSB in which lightly contaminated insulation blankets removed from an in-service MD-11 were tested, no appreciable differences were noted in the flammability characteristics of the material.

The aviation industry has yet to quantify the impact of contamination on the continuing airworthiness of insulation blankets. However, research has shown a connection between flammability and contamination. The following extract is from the FAA's Flight Standards Information Bulletin 00-09, issued 28 September 2000 and expired 30 September 2001 entitled Special Emphasis Inspection on Contamination of Thermal/Acoustic Insulation:

Research data has shown, however, that the flammability of most materials can change if the materials are contaminated. Contamination may be in the form of lint, dust, grease, etc., and can increase the material's susceptibility to ignition and flame propagation.

During the examination of several Swissair MD-11 aircraft during the investigation, contamination (as described in Section 1.12.5) was observed on items such as light fixtures and wire harnesses. Contamination was also noted on insulation blanket cover materials within the fire-damaged area; however, little or no contamination was evident in the areas above the cockpit ceiling.

[69]    Pieces of recovered insulation blanket were labelled "Insulfab 350, DMS 2072K Type 2, Class 1, Grade A," which were constructed using a metallized polythylene terephthalate (PET) polyester film.

[70]    Pieces of recovered insulation blanket were labelled "Orcofilm AN-34, DMS 2312 Type 2," which were constructed using a metallized polyvinyl fluoride polyester film.

[71]    On 15 March 1999, Boeing issued an All Operator Letter (AOL) entitled Effect of Contamination on Insulation Blankets. The AOL indicates that contaminants can act as a fuel and suggests that operators implement action to inspect and clean insulation blankets during appropriate scheduled maintenance periods. The following is an extract from that AOL:

Test results indicate that contaminants act as fuel for a localized ignition source and may negate the self-extinguishing properties of the cover materials. Layers of contaminants can build-up over time. As a result, we suggest that operators implement action to inspect and clean insulation blankets during appropriate scheduled maintenance periods.

[72]    The following extract is from a document produced by The Mexmil Company, as presented at the International Aircraft Fire and Cabin Safety Conference, 16 to 20 November 1998:

Used aircraft fuselage thermal/acoustical insulation blankets were procured from a jet in passenger service for at least 10 years. Their flammability and physical properties were tested for comparison to those of identical, newly fabricated blankets. In part, the findings of this study were:
  • aged blankets showed a greater propensity to propagate flames and produce more smoke;
  • surface contamination with corrosion inhibiting compounds likely contributes to their increased flammability; there are many spaces in aircraft, including some large areas within transport category aircraft, that are seldom inspected. Such spaces can become contaminated, particularly with dust. Maintenance programs may limit such contamination, but it is unlikely that contamination can be completely eliminated from in-service aircraft.

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