Aviation Investigation Report A98H0003
4.2.1 Thermal Acoustic Insulation Materials
- 126.96.36.199 - Other Thermal Acoustic Insulation Materials at Risk
- 188.8.131.52 - Proposed Certification Standard for Thermal Acoustic Insulation Materials
Since the beginning of this investigation, the aviation industry's understanding of the flammability characteristics of thermal acoustic insulation materials has advanced considerably. The recognition that MPET-covered insulation blankets are flammable and provided the main source of fuel in the SR 111 in-flight fire was significant. Extensive flammability testing determined that such blankets are susceptible to being ignited by small ignition sources, such as electrical arcing or sparking and will propagate a fire. Consequently, the FAA required that these blankets be removed from US-registered aircraft, and accelerated work to develop an improved flammability test for the certification of all thermal acoustic insulation materials.
Occurrence data confirms that some thermal acoustic insulation materials, other than MPET-covered insulation blankets, have been involved in aircraft fires that were ignited by electrical sources. FAA research revealed that these other thermal acoustic insulation materials, although more difficult to ignite, exhibit similar flammability characteristics once ignited. The flammability test that was used to certify all such materials (i.e., the vertical Bunsen burner test) was designed to determine whether the material would ignite from a small ignition source, such as an electrical arcing event, and extinguish within a predetermined flame time and burn length. All such materials were approved for use in aircraft because, once ignited, they self-extinguished within a predetermined flame time and burn length. The FAA's Radiant Panel Test (RPT) certifies materials using similar, albeit much more stringent, criteria.
The FAA has tested a representative sample of thermal acoustic insulation materials currently in use in the aviation industry and has determined that approximately two-thirds failed the RPT. Because the RPT effectively fails materials that could be ignited from a small ignition source, including an arc or spark, then potentially, these failed materials could exhibit such inappropriate characteristics while in-service. If the RPT is ultimately approved, any materials that fail the RPT would not be acceptable for use in any future aircraft manufacture or repair. However, unlike the case with MPET-covered insulation blankets, there is no indication that regulatory authorities will mandate a wholesale removal, from existing aircraft, of those other in-service thermal acoustic insulation materials that failed the RPT.
Additionally, since smoke generation and toxicity limits have never been established for thermal acoustic insulation materials, the associated risks have not been quantified. Such risks would likely be a factor if these flammable materials become involved in an in-flight fire. It has been suggested that, once the RPT is adopted, the "zero burn" feature of the RPT will result in the eventual elimination of flammable thermal acoustic insulation material in aircraft, and therefore, measuring a material's smoke generation and toxicity levels, as part of the certification process, is unnecessary. However, under the present approach, mitigation of the risks associated with these flammable materials will not be accomplished until the existing fleet of aircraft is replaced. Therefore, known flammable materials will exist for decades in thousands of aircraft worldwide.
The in-flight fire risks associated with MPET-covered insulation blankets have largely been mitigated. However, there are other thermal acoustic insulation materials that once ignited, exhibit similar flammability characteristics to MPET-covered insulation blankets, and have failed the RPT. Although these materials exist in many aircraft, as of this report's publication date, no mitigation strategy has been undertaken to address the known associated risks. Therefore, the Board recommends that:
Regulatory authorities quantify and mitigate the risks associated with in-service thermal acoustic insulation materials that have failed the Radiant Panel Test.
Assessment/Reassessment Rating: Unsatisfactory
The FAA has proposed a rule that would replace the existing vertical Bunsen burner test with the RPT to evaluate fire ignition and propagation characteristics of all thermal acoustic insulation materials. During its validation of the RPT, the FAA reported that only 25 to 35 per cent of the various insulation blanket cover materials would pass the RPT. The proposed test has been widely accepted as a major improvement over the previous test in that it effectively imposes a "zero burn" criterion for all thermal acoustic insulation materials. Although the test would be required for all thermal acoustic insulation materials, and appears to be a better discriminator of materials that exhibit inappropriate flammability characteristics, the design of the RPT contains some inherent limitations.
The RPT is designed to expose the test specimen to a small fire-in-progress scenario that sets higher ignition and propagation threshold "pass" requirements. However, there are concerns about whether the current RPT suitably addresses the following key issues:
- Although the FAA believes that a test specimen's orientation is an important factor in determining its propensity to be ignited and propagate a fire, the RPT only requires that a specimen be oriented horizontally;
- The RPT has its origins in the American Society for Testing and Materials E648 test, which requires that the test specimen be pre-heated prior to the application of the flame. Although the FAA recognizes the benefits of pre-heating test specimens because of the deleterious effects on the thin-film covered thermal acoustic insulation materials, the RPT does not impose this pre-heat condition; and
- The RPT requires the testing of three specimens that include all those materials used in the construction of insulation blankets (including batting, film, scrim, tape, etc.). However, it does not indicate how the flammability characteristics of the component materials are to be tested in the various permutations and combinations while only requiring that three specimens be tested.
Also, the Board is aware of initiatives by the FAA to design the RPT to account for potential degradation in the flammability characteristics of materials after they are exposed to their intended operating environment. The FAA has recognized that most aircraft in-service have insulation blankets with varying degrees of surface contamination, and that experience has shown that such contamination cannot be fully avoided. Therefore, one goal of the testing is to develop an appropriate evaluation procedure that can account for realistic in-service conditions.
Because the issues listed above are not addressed, it is unclear how the RPT would effectively identify all thermal acoustic insulation materials that may exhibit inappropriate flammability characteristics. Rather, it appears that the RPT is a single certification test for thermal acoustic insulation materials, which under certain conditions (such as conditions that do not involve pre-heating), results in an effective flammability test for thin-film-covered insulation blanket materials.
By developing the RPT, the FAA has successfully designed a single certification test that, while a major improvement over the vertical Bunsen burner test, may not successfully evaluate the performance of all types of thermal acoustic insulation materials under representative conditions. Given these limitations of the FAA's proposed RPT, the Board recommends that:
Regulatory authorities develop a test regime that will effectively prevent the certification of any thermal acoustic insulation materials that, based on realistic ignition scenarios, would sustain or propagate a fire.
Assessment/Reassessment Rating: Satisfactory Intent
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