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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft 5 Conclusions and Recommendations Research Opportunities Aircraft interiors are complex systems that include a number of components—visible items such as flooring, seats, lavatories, ceilings, sidewalls, stowage bins, bag racks, closets, and windows, and items that are not visible to passengers such as ducting, wiring, insulation blankets, and supporting structures. Polymeric materials are predominant, appearing in a wide range of product forms including molded sheet or shapes, composite-faced honeycomb sandwich, textile fibers (fabrics or carpets), foams, sealants, and adhesives. Interiors currently contain materials of varying fire resistance, selected for their particular application and a variety of additional factors such as availability, cost, producibility, and a balance of other useful properties. Environmental safety and health concerns during processing, fabrication, transport, use, and disposal (including ability to recycle), must also be considered. The development of improved fire-resistant materials must consider all these system-related factors in addition to the materials-related flammability characteristics such as reduced heat release, delayed or no ignition, reduced ability to support combustion, and reduced smoke and toxicity. Materials used in the production of current aircraft interiors, with some exceptions, tend to have better fire resistance than materials used in other transportation systems.1 Regulatory requirements have been significant driving forces in the optimization of fire resistant polymers and the development of required product forms for aircraft interior applications. Independent programs pursued by industry have also resulted in essentially a new generation of materials that found application in the 747, DC-10, and L-1011, and then a second generation of materials used for the 767, 757, A300-600, and A310. The FAA's heat and smoke release regulations drove improvements to the second-generation materials and to the application of new materials such as more fire-resistant thermoplastics to satisfy specific application needs. The committee believes that long-term, focused research in fire-resistant polymeric materials can lead to significant improvements in fire performance and safety. To support the development of such materials, advances are required in the understanding and analytical modeling of aircraft fire scenarios, polymer combustion, small-scale characterization tests, and fire-hazard assessments. In addition to the materials' properties, the development process must address the needs of user of the materials. These include the processing and production capabilities of the materials suppliers and aircraft manufacturers; the ability to meet the design, performance, comfort, and aesthetic demands of aircraft interior applications; and compliance with environmental, health, and safety regulations and practices in the manufacture, use, and disposal of aircraft interior components. The committee focus was on materials technology and enabling design, manufacturing, testing, and modeling capabilities. A comprehensive research program with the goal of improving survival of aircraft accidents would also include aspects of fire- safety and-suppression systems, human factors and behavior in emergencies, and sensor and control development for accident avoidance. While the committee is confident that significant improvements can be realized in materials performance, the FAA goal of an "order-of-magnitude" improvement in fire resistance is difficult to define because of the multitude of performance metrics and fire scenarios that need to be considered and evaluated. Establishing specific performance goals for materials research based on the current understanding of materials combustion and aircraft fire scenarios is problematic because the data needed to relate materials performance and configurations to observed fire scenarios are not available. The committee recommends that materials performance goals for long-term research be established using hazard and risk assessment techniques. These techniques require experimental data from appropriate small-scale tests in conjunction with fire models to predict the expected fire performance and assess the probability of occurrence under realistic conditions, followed by validation tests in the intermediate-and full-scale regime. 1 Mass transit standards vary and are often difficult to compare with aircraft standards (NRC, 1991). For example, the New York Transit Authority has taken extreme measures to provide minimal fuel to sustain a fire. The October 29, 1995, subway fire in Baku, Azerbaijan, which resulted in over 300 fatalities, provides an example where fire resistance standards were not adequate.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft MATERIALS The committee has identified three key research directions for the development of improved fire-resistant materials: Modification of specialty polymers including thermoplastics such as polyetheretherketone, polyetherimide, polyphenylene sulfide, and polysulfone and thermosets such as cyanate esters, bismaleimides, polyimides, and polybenzimidizole. This approach may provide the best performance in the near term (<10 years). Development of new, high-performance, thermally stable materials including organic/inorganic systems, copolymers, polymer blends and alloys, and glasses and ceramics. These materials have the potential for the best performance in the long term (> 10 years). Modification of existing engineering polymers including thermoplastics such as polycarbonates, nylons, and polyethyleneterephthalate and thermosets such as phenolics and polyesters. While it is not clear that this approach would lead to the significant improvement in performance sought, this approach may result in significant cost reductions. A basic scientific understanding of char and intumescence on flammability is crucial to the development of improved materials. Research in char formation should include structural characterization and mechanical behavior (durability) and its relationships to exposure atmosphere, heating rate, chemical derivatization, additives, and coatings. Also the effect of char formation on toxicity needs to be characterized. The two general technical directions for polymer materials development to improve fire and smoke resistance identified by the committee are incorporation of additives in polymers and synthesis of thermally stable, tim-resistant polymers. Particularly promising approaches are discussed in detail in Chapter 4. These include thin laminated or co-extruded films and blends, coatings and additives (including intumescents), phase-change or temperature sensitive materials, organic/inorganic polymer blends, polymer blends utilizing a high char-forming polymer as an additive, and polymer modifications. Additive approaches include volatile-phase active flame retardants that inhibit the combustion process, condensed-phase active flame retardants that lead to char or intumescence, flame retardants that endothermically lose non-toxic volatile components, heat-sink additives, toxicant suppressants, and combinations of additives that take advantage of synergistic effects (i.e., multiple additives with differing but cooperative modes of activity in optimized combinations). Recommendations: Perform research to improve the fundamental understanding of polymer combustion, including thermal degradation, char formation, intumescence, toxic gas production, and heat effects. Place special emphasis on the characterization of char and intumescence processes. Investigate new additive approaches that allow for significant improvements in fire resistance and reduced toxic gas production in current materials. Facilitate the development of new or modified polymers with significantly improved resistance to ignition and flame spread. Emphasize the modification of existing specialty polymers to obtain desired properties and the development of new thermally stable polymers or blends. Evaluate and prioritize research and technological development efforts to ensure that the new materials will meet end-use requirements. Issues to be considered include costs; the contemporaneous processing and production capabilities of the materials and aircraft industries; ability to meet the design, performance, comfort, and aesthetic demands of aircraft interior applications; and compliance with environmental and health and safety regulations and practices. COMPONENT DESIGN AND MANUFACTURING New fire-resistant materials are of little practical value for aircraft interior use if the industrial processing technologies required to manufacture parts are not fully developed and broad-based. Short-and long-term strategies should be developed to characterize new material opportunities for compatibility with existing processes, as well as determining needs for future designs and manufacturing technologies. Short-to mid-range strategies should focus on researching materials that can be produced with existing tooling and manufacturing processes. Long-term strategies should evaluate both materials that can be processed with today's technologies as well as with future technologies. Where improved fire performance can only be achieved with materials requiring new manufacturing processes, materials research and manufacturing process development should be conducted concurrently to ensure smooth implementation. Research should also be aimed toward developing material constructions with equivalent or lower-weight and simplified processing requirements compared with currently used materials. Acceptance and utilization of new materials would be greatly enhanced where manufacturing and in-use performance advantages are readily achieved and demonstrated. For example, new materials and manufacturing approaches may
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft be able to reduce the number of processing steps and cure time to relieve the current labor-intensive fabrication of honeycomb core sandwich panels. Enhanced thermoplastics could be developed for rapid manufacturing while meeting the other performance requirements described in Chapter 2. Such materials may be easier to rework and recycle and can be more environmentally "benign." New modular design technologies should be pursued to reduce the required number of parts by integrating components. Minor changes to component designs may also yield improved fire performance, Lightweight films and coatings with improved fire resistance that can be easily integrated into current component constructions such as interior sandwich panels or insulation blankets should be investigated. Recommendations: Prioritize materials research opportunities in terms of compatibility with existing tooling and manufacturing processes. Short-to mid-range programs should focus on materials systems highly compatible with existing manufacturing technologies for a smoother introduction into production. In long-range development where new manufacturing processes are needed, materials research and manufacturing process development should be conducted concurrently to ensure smooth implementation. Investigate innovative design and processing concepts such as modular design, fire-resistant films and coatings, new thermoset composite materials and manufacturing approaches to reduce the number of processing steps and cycle times, and expanded use of thermoplastics. These concepts could provide improved fire resistance while reducing manufacturing costs. FIRE SCENARIOS A complete understanding of aircraft fires and the responses of materials and components in these fires is required to establish appropriate performance goals and evaluation criteria for new fire-resistant materials. Based on prior experience, two basic fire scenarios have been identified: post-crash fires involving (potentially large) quantities of aviation fuel from ruptured fuel tanks and in-flight fires involving only interior cabin furnishings and passenger-specific items. These scenarios, described in detail in Chapter 3, provide the basis for establishing fire performance behavior and criteria for new materials. However, new aircraft configurations may be significantly different from past designs, and the response of aircraft interiors in these fire scenarios depends on the details of the design. Thus, each aircraft configuration must be analyzed to assess its response. Examples of potential changes that may affect fire scenarios are the variable cross-section, aerodynamically blended fuselage of the proposed High Speed Civil Transport and multiple-deck passenger cabins of very large (800 passenger) sub-sonic transports.2 Radical changes in aircraft designs, such as a flying wing, would have an even greater effect on fire scenarios but are far less likely to reach production within the 10–20 year time frame considered in this report. As discussed in Chapter 3, there are several variations of post-crash, external fuel-fire scenarios that need to be considered. Different factors control ignition and fire spread for each variation: Case 1: openings in the fuselage and only flame radiation enters. The ignition and flame-spread characteristics of the interior components under the external radiation are critical. Case 2: openings in the fuselage allowing flames and fuel degradation products (i.e., hot gases and smoke) to enter. Here the upper layer of the cabin can quickly become vitiated (contaminated with degradation products). Flame spread along the ceiling and toxicity characteristics of combustion products in vitiated atmospheres are critical to survivability in areas away from the openings. Case 3: no hull openings. The heating of the airplane skin will eventually overheat the back side of the interior wall panels and insulation, resulting in some degree of aerobic pyrolysis or burnthrough of the skin and insulation systems. Fuel flammability can overwhelm post-crash fire scenarios. The heat and fire-spread characteristics of a fuel fire can cause materials with outstanding fire resistance to burn readily. Also, as described in Case 2 above, the smoke and hot combustion products of the fuel fire can represent a severe hazard even if interior furnishings do not become involved in the fire. For these reasons, while beyond the scope of the committee's deliberations, the reduction of the fire hazard of fuel is critical in improving survivability in post-crash fires. In the past, in-flight interior fires have very rarely developed into accident scenarios. Those within the passenger compartment have been detected and extinguished before posing a significant threat and most that began in or around lavatories either were detected and extinguished or self-extinguished. Therefore, in-flight fires in accessible areas within the aircraft interior were not considered in this study. However, in-flight fires in inaccessible areas can be a serious 2 The potential for very large transport aircraft presents challenges in design for fire safety. These challenges include placement and number of emergency exits, effects on detector and extinguishing systems, fire stops and compartment design, and venting.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft concern because of the potentially long periods (up to three hours) before passengers can be evacuated. The number of accidents that began as a fire in a cargo compartment is, relatively speaking, extremely small. Nevertheless, recent regulatory upgrades of cargo compartment liners require them to perform as substantial fire barriers to contain possible fires from spreading into the passenger compartment. The role of the toxicity of combustion products in aircraft accidents needs to be better defined. In post-crash, fuel-fed fire scenarios, smoke originates from at least two sources: burning materials inside the cabin (this includes not only standard cabin furnishings, but also combustibles associated with passengers such as carry-on luggage, clothing, and purses), and burning jet fuel located in areas where the smoke produced may enter the cabin through hull openings or various doors or escape hatches which must be opened for passenger evacuation. The FAA determined in its work during the development of the heat release regulation that the toxicity hazard from burning standard cabin furnishings did not become significant until flashover occurred. However, when flashover occurred in the tests, the contribution to hazard from heat alone was shown to become nonsurvivable; hence the contribution of toxicity was essentially not an issue for survivability with the materials tested in such a scenario. The FAA has not investigated the toxic hazard from the combustion products of jet fuel that can enter the cabin during a post-crash fuel-fed fire scenario. This is an important gap in the current knowledge that must be filled. Testing and Evaluation To perform an adequate fire-hazard analysis or flammability assessment of a material, several materials characteristics must be integrated. Experimental data from appropriate small-scale tests should be used in conjunction with fire models to scale the expected fire performance up to realistic behavior, followed by validation tests in intermediate-and full-scale regimes. This type of assessment is sensitive to the end-use application, type of material, and actual fire threats. Adaptation of current test methods, and, in many cases, new small-scale test methods, are needed to evaluate fire performance characteristics of materials for specified aircraft interior situations and to provide property data for use by modelers to predict component fire performance in expected large-scale fire scenarios. Rather than being used exclusively as pass/fail screening tests, small-scale tests should be used to measure flammability properties of the materials that can be used as input to theoretical models to predict fire hazard. This process requires enhanced interaction between the experimentalist and the modeler to establish that the test procedures are designed to obtain the parameters that the models require. A better understanding of the performance, limitations, and operating principles of existing test equipment and the development of new and better test methods are needed. Toxicity tests need to be either developed, or currently available procedures need to be appropriately modified, to simulate the fire scenarios of concern in aircraft. Materials present in a fire, including standard cabin furnishings, jet fuel, and passenger items should be decomposed to produce smoke in a manner likely to occur in the fire scenario(s). The gases currently considered in N-gas models—CO, CO2, HCN, low levels of O2, HCI, HBr, and NO2—should be measured and used to predict the LC50 values of alternative materials. This prediction can be checked using bioassay experiments to ensure that no toxic gas other than those expected are produced. This approach can be used to compare the toxicity of various experimental materials that could serve in the same end-use. Additional tests procedures need to be developed to examine incapacitation, mutagenicity, carcinogenicity, and teratogenicity from exposures to combustion products. After small-scale tests, materials and components should be evaluated in tests of increasing scale. An example of such an approach is the military standard for composite materials for submarine applications (DOD, 1991). This standard included a burnthrough fire test to assess the fire resistance of a material and to provide comparison information on its fire containment and propagation (fire, smoke, and fire gases); a quarter-scale fire test to determine the flashover potential of materials in an enclosure when subjected to a fire exposure; a large-scale open environment test to test materials at full size in their intended application under a controlled laboratory fire exposure to determine fire tolerance or ease of extinguishment; and a large-scale pressurizable fire test to test materials in their intended application in a simulated submarine environment under a controlled laboratory fire exposure. The development of a materials fire-test database can provide a framework to establish performance criteria, evaluate new materials, and predict materials behavior in-service. The steps involved in database development are (1) categorizing and cross-referencing test methods and characteristics and identifying the fire parameters needed for hazard assessment and modeling; (2) compiling existing fire characterization data; (3) obtaining relevant fire-test data through testing programs as they become available; and (4) developing new test methods where needed to provide additional data not available from current methods. In order to make the database complete, flammability characteristics that must be included are heat of combustion, pyrolysis rate, char depth, heat release rate, char yield, thermal properties, surface temperature at ignition, and heat of gasification. Flame-spread models require good empirical data for
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft these parameters in order to make accurate predictions for time-dependent behavior of materials. Other areas of needed research are material fire performance under unusual conditions such as low oxygen concentrations and low atmospheric pressure or under wind-assisted conditions. Also, current methods do not provide ways to accelerate aging of materials to evaluate retention of low flammability characteristics. To understand how material systems perform in different configurations and orientations, additional data analysis methodologies are needed (e.g., more heat transfer experimentation, especially for ceiling configuration). Once confidence has been gained on the completeness and accuracy of the materials fire-test database, fire performance of aircraft interior materials can be measured through the judicious implementation of these input parameters in fire models to predict behavior such as rate of flame spread or time to flashover. Recommendations: Develop the science base for small-scale fire performance and toxicity tests, based on expected fire scenarios and verified with full-scale tests, to provide meaningful property data for modeling and materials evaluation. Develop a database of materials fire performance properties to provide a means to establish performance criteria, evaluate new materials, and predict materials behavior in aircraft applications correlatable with expected fire scenarios. Support technology scale-up through testing on an increasing scale, from small-scale through full-scale testing. Modeling An integrated modeling capability for aircraft interior designers could allow the estimation of the performance benefit of various choices of fire-resistant materials and components in aircraft interior applications. Analytical models, ranging in scale from molecular to full scale, are needed to support the development and evaluation of new fire-resistant materials. Models are used to predict materials performance in fires and to assess the fire hazard. Thermal degradation models that include crosslinking, cyclization, aromatization, and network formation could be used as tools to determine ways to enhance the thermal stability of polymers and to promote char formation during polymer degradation. Intumescent char models based on the formation and growth of bubbles, swelling, polymer melt behavior, and carbonization may be used as tools in optimization of intumescent materials or coatings. The models should be applicable to engineering polymers, specialty polymers, polymers with fire-retardant or toxicant suppressing additives, and polymer blends. A fire growth model, including submodels to predict ignition, flame spread, heating/pyrolysis/burning, and flame and surface heat transfer, is needed. Current models have greater capabilities to predict smoke spread through multiple compartments than flame spread in both horizontal and vertical directions. Sources that include both thermal or nonflaming and piloted ignition and heating must be considered since both methods produce different modes of preparation of the surface for burning, different products of combustion, different levels of fire intensity, and require different forms of passive fire protection to counteract the source. Once ignited, materials undergo combustion depending on the mode of ignition (energy and exposure time), oxygen availability, and physical and chemical material characteristics. There are two different types of models: zone models and field models. Zone models are not yet able to include all necessary fuselage characteristics, such as shape (especially ceiling shape), and upper-level fuels, such as combustible ceiling panels and items contained in baggage compartments. Relatively straightforward additions to current zone models should be incorporated. The needed additions are simple in part because, unlike buildings, airplane interiors are now very similar. Current field models solve the Navier-Stokes equations with some type of turbulence model. They need more accurate and computationally efficient radiative heat transfer calculation schemes and turbulent buoyant flow-field calculations, as well as a means to include more realistic condensed-phase and gas-phase chemical reactions. An area that appears to be inadequate at this time is modeling flame-heat transfer. A good combustion submodel that allows self-consistent burning will provide a global fire model that is less dependent on empirical input for predicting time-dependent heat release rate or mass-loss rate. The advantage of developing a combustion submodel of this type is that the predictive capability of the fire model relies less on the availability of specific material performance data. For completeness, the mechanisms by which agents suppress combustion or toxic gas production need good model development. Whether acting by physical or chemical means, the mechanisms of how suppression agents work needs to be understood so more effective, environmentally safe fire-suppression agents can be developed. This is especially important since the manufacture and use of most highly effective halo-carbon fire-extinguishing agents is being curtailed because they cause depletion of stratospheric ozone. The same problems associated with modeling ignition have been encountered in suppression modeling. Steady-state conditions do not exist, and changes could be occurring on a micro scale, adding complexity to the modeling process. Hazard assessment models need to be specially designed to address the aircraft interior, the fire scenarios of interest. Current models do not account for aircraft interior configuration. The combustion of the new materials would need to be tested to generate data for input into the models.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft The evacuation of aircraft is very different from that expected in a building or residence and those differences need to be incorporated into hazard models. As the fire resistance of interior materials improves, toxicity models may need to be expanded to determine the effects on accident survivors of gases likely to be generated if the new fire-safe materials thermally decompose. Other potential effects that should be evaluated are releases of hot water vapor, particulates, and free radicals; the adverse effects of heat on the human body; and the effects of simultaneous exposure to toxic gases and heat. In the long term, the effects of factors such as physical exertion and alcohol may also be incorporated. Instead of test animals (e.g., rats, mice), increased use of in vitro endpoints should also be investigated. Long-term health effects, such as mutagenicity and carcinogenicity, from short-term exposures need to be studied. Finally, the models under development must be validated to gain the confidence of the design community. This requires a closely coordinated research effort between theoretical model development and intermediate and large-scale validation testing. More emphasis needs to be placed on requiring intermediate and large-scale testing to verify small-scale data and to refine the understanding of the effects of size and configuration of interior components on the fire performance. Recommendations: Develop basic thermal degradation models that are applicable to engineering and specialty polymers and include crosslinking, cyclization, aromatization, and network formation to aid the understanding of polymer stability and char formation. Include both char characteristics and evolved gaseous product properties as key model parameters. Develop intumescent char models based on the formation and growth of bubbles, swelling, polymer melt behavior, and carbonization. Develop an integrated modeling capability that will allow the estimation of the performance benefit of various choices of fire-resistant materials and components in aircraft interior applications. Work is needed to develop fire-growth, toxicity, and hazard assessment models relating to aircraft fire scenarios. Long-Term Research Program The committee believes that the goals of the FAA's research program to develop significant order-of-magnitude improvements in materials fire performance cannot be met with incremental advances or near-term regulatory activity. Rather, substantial advances based on a fundamental understanding of polymer combustion, on accurate aircraft fire scenarios, and on the systematic development of materials technology improvements are required. These advancements require a long-term commitment on the part of the FAA working with the aircraft and materials industries and research laboratories. The uncertainty of new commercial programs, the cost of qualification and certification, and the long time-to-market for new materials tends to discourage suppliers from embarking on materials development efforts. The size of the potential market for materials for use in aircraft interior components often does not justify the expense to the suppliers of development and qualification. Thus, it is important to develop alternate markets for new materials and to apply technology developments from other industries. Many of the developments that arise from this research will be unique to the issue of commercial aircraft interior fire safety. However, advances in the understanding of polymer combustion, new materials and additive technology, and testing and modeling methods may have applicability to fire safety in other transportation systems such as submarines, ships, mass-transportation systems, automobiles, and buses, as well as commercial and residential buildings. If this long-term research effort is sustained and a coordinated, parallel effort persists in these related areas, significant advances will be made in the understanding of materials fire safety not only for commercial aircraft interiors but also in many other areas where fire safety is a concern. Recommendations: Sustain the effort to develop significantly improved fire-resistant materials as a long-term research program, with clearly stated goals, plans for systematic technology development, and stable financial commitment. Continue to follow developments in fire safety in the materials and aerospace industries, as well as in related industries. Coordinate within the U.S. Department of Transportation and with other federal agencies conducting related research, including the National Aeronautics and Space Administration, the departments of Defense, Energy, Transportation, Commerce, and the National Science Foundation.
Representative terms from entire chapter: