Aviation Investigation Report A98H0003
2.3.1 Flammability of Materials
The most significant deficiency in the chain of events that resulted in the crash of SR 111 was the presence of flammable materials that allowed the fire to ignite and propagate. Testing conducted during the investigation showed that several materials located in the heat-damaged area were flammable, even though they met regulatory standards for flammability. The metallized polyethyelene terephthalate (MPET)covering material on the thermal acoustic insulation blankets (insulation blankets) used in the aircraft was flammable. This was the most significant source of the combustible materials that contributed to the fire. The MPET-covered insulation blanket was also most likely the first material to ignite. Other materials in the area of the fire damage were also found to be combustible and to have contributed to the propagation and intensity of the fire. These materials included silicone elastomeric end caps; hook-and-loop fasteners; foams; adhesives; and different kinds of splicing tapes used in the construction, installation, and repair of insulation blankets.
The certification testing procedures mandated under flammability standards that existed at the time of the occurrence were not sufficiently stringent or comprehensive to adequately represent the full range of potential ignition sources. Nor did the testing procedures replicate the behaviour of the materials when installed in combination, or in various locations and orientations, as they are found in typical aircraft installations and realistic operating environments. The lack of adequate standards allowed materials to be approved for use in aircraft, even though they could be ignited and propagate flame.
Two primary factors shaped the flammability standards in place at the time of the occurrence.
- The approach taken by the Federal Aviation Administration (FAA) in the mid-1970s to concentrate its fire prevention efforts in the following two areas: improved cabin interior materials and higher standards for materials in designated fire zones.
- A lower priority assigned to fire threats in other areas. The non-fire-zone hidden areas were viewed as benign from a fire hazard perspective, as they were seen to be free of the combination of the two elements needed for a fire: a potential ignition source and flammable materials.
The ground fire incidents involving the MPET and non-metallized polyethylene terephthalate insulation blanket cover material that led McDonnell Douglas to reassess its flammability testing of insulation blankets did not trigger mitigating action by regulators. Testing by the manufacturers, the Civil Aviation Administration of China (CAAC), and the FAA showed that these materials could ignite and burn; however, the FAA's follow-up on this issue did not include mandating action to mitigate the potential fire threat. Although McDonnell Douglas stopped using MPET-covered insulation blankets in its production aircraft, and issued a Service Bulletin recommending that operators replace it with a different material, neither the FAA nor other airworthiness authorities required its removal from in-service aircraft until after the release of the safety recommendations made by the TSB following the crash of SR 111.
In 1996, the CAAC pointed to the flammability of the materials as a safety issue in its investigations into two separate in-flight fire occurrences. However, other investigations involving aircraft fires in which MPET-covered insulation blankets were involved typically focused on the ignition source rather than the flammable material. Those investigations did not highlight the safety deficiency posed by the flammable materials.
Ultimately, the in-service flammability performance of MPET-covered insulation blankets prompted an FAA-led research program to quantify the deficiency and develop a specific flammability test for thermal acoustic insulation materials. More than two years of research resulted in the FAA proposing the adoption of the more stringent test, entitled the Radiant Panel Test (RPT). This test exposes the materials to a more realistic in-flight fire scenario and effectively imposes a "zero" burn requirement. Validation of the RPT confirmed that MPET-covered insulation blankets were highly susceptible to flame propagation when ignition occurs from a small ignition source. This confirmation prompted the FAA to issue Airworthiness Directives (AD), applicable to US-registered aircraft, that state that "a determination be made of whether, and at what locations, metallized polyethylene terephthalate (MPET) insulation blankets are installed, and replacement of MPET insulation blankets with new insulation blankets" and "[t]he actions specified by this AD are intended to ensure that insulation blankets constructed of MPET are removed from the fuselage." Although these ADs are enforceable only with respect to US-registered aircraft, most other regulatory authorities throughout the world generally follow the FAA's lead and endorse FAA ADs.
While MPET-covered insulation blankets are identified as the most vulnerable, the research also established that many other widely used thermal acoustic insulation cover materials did not meet the requirements of the RPT. Furthermore, the ADs that call for the removal of MPET-covered insulation blankets warn that other cover materials, although harder to ignite, burn in a similar manner as MPET-covered material. The TSB has expressed concern about the flammability characteristics of other materials that were approved under the same testing procedures used to certify the MPET. Many of these cover materials have been shown to be flammable in subsequent testing.
Most aircraft crews are likely unaware that under certain conditions, a fire could ignite significant flammable materials in hidden areas of aircraft and spread rapidly. Had the pilots been aware that flammable materials were present in the attic space of the MD-11, this knowledge might have affected their evaluation of the source of the odour and smoke.
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