For the purposes of this study, the interior of an aircraft is considered to be everything that is contained inside the pressure shell, that is, the pressurized part of the aircraft fuselage. The regulatory requirements of Federal Aviation Requirements (FAR), Part 25, that apply to interiors fall in FAR 25.853, Compartment Interiors, and FAR 25.855, Cargo or Baggage Compartments. Although FAR 25.853 has the introductory terms ''For each compartment occupied by the crew or passengers, the following applies,'' items that ate not strictly in the occupied compartment (i.e., are outside the cabin liners and not visible to either crew or passengers, such as "electrical conduit, thermal and acoustical insulation and insulation covering, air ducting," are specifically cited and the regulatory requirements are also applied to them. Interior cabin liner components are identified in Figure 2-1.
An aircraft interior is designed to meet the requirements of:
the FAA and other regulatory agencies,
airline passengers, crew, and
the aircraft manufacturers.
There are minimum requirements that emanate from these four groups, which together comprise the design criteria. Issues to consider in the development of combined (inclusive) design criteria for aircraft interiors are shown schematically in Figure 2-2.
The safety criteria include the FAA regulatory mandates, which address only safety and axe largely quantitative. However, there are other, nonregulatory requirements such as passenger comfort level that are difficult to quantify, which complicates the task of the designers. Aircraft interior design is further complicated by the fact that many of these needs compete with each other and thus trade-offs are necessary.
Once the design of a part has been established by design engineering organizations, and once drawings describing the design and manufacture are released to the manufacturing organizations, many business processes are activated to car out acquisition of materials (inventory), tools, facilities, and manpower. If a subsequent change is made to the design, all the manufacturing planning is also subject to change, which can be time consuming and costly and creates the potential for a substantial economic penalty. There is, therefore, a strong priority assigned to designing parts "right the first time."
The current state of the art for materials used to make parts that satisfy the design criteria and other requirements fall into several main categories or families. Materials categories that could be used to fabricate more fire-resistant interiors would be subject to the same selection and use criteria.
Currently, most of the vertical and ceiling surfaces of aircraft are comprised of sandwich panels fabricated from face sheets of phenolic resin and fiberglass or carbon fiber reinforcement, and a polyaramid (Nomex®) core. These panels are covered with highly formable decorative thermoplastic films that are printed in a variety of complex patterns and
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft 2 Design and Function Requirements for Aircraft Interior Materials For the purposes of this study, the interior of an aircraft is considered to be everything that is contained inside the pressure shell, that is, the pressurized part of the aircraft fuselage. The regulatory requirements of Federal Aviation Requirements (FAR), Part 25, that apply to interiors fall in FAR 25.853, Compartment Interiors, and FAR 25.855, Cargo or Baggage Compartments. Although FAR 25.853 has the introductory terms ''For each compartment occupied by the crew or passengers, the following applies,'' items that ate not strictly in the occupied compartment (i.e., are outside the cabin liners and not visible to either crew or passengers, such as "electrical conduit, thermal and acoustical insulation and insulation covering, air ducting," are specifically cited and the regulatory requirements are also applied to them. Interior cabin liner components are identified in Figure 2-1. An aircraft interior is designed to meet the requirements of: the FAA and other regulatory agencies, the airlines, airline passengers, crew, and the aircraft manufacturers. There are minimum requirements that emanate from these four groups, which together comprise the design criteria. Issues to consider in the development of combined (inclusive) design criteria for aircraft interiors are shown schematically in Figure 2-2. The safety criteria include the FAA regulatory mandates, which address only safety and axe largely quantitative. However, there are other, nonregulatory requirements such as passenger comfort level that are difficult to quantify, which complicates the task of the designers. Aircraft interior design is further complicated by the fact that many of these needs compete with each other and thus trade-offs are necessary. Once the design of a part has been established by design engineering organizations, and once drawings describing the design and manufacture are released to the manufacturing organizations, many business processes are activated to car out acquisition of materials (inventory), tools, facilities, and manpower. If a subsequent change is made to the design, all the manufacturing planning is also subject to change, which can be time consuming and costly and creates the potential for a substantial economic penalty. There is, therefore, a strong priority assigned to designing parts "right the first time." MATERIALS OF CONSTRUCTION The current state of the art for materials used to make parts that satisfy the design criteria and other requirements fall into several main categories or families. Materials categories that could be used to fabricate more fire-resistant interiors would be subject to the same selection and use criteria. Currently, most of the vertical and ceiling surfaces of aircraft are comprised of sandwich panels fabricated from face sheets of phenolic resin and fiberglass or carbon fiber reinforcement, and a polyaramid (Nomex®) core. These panels are covered with highly formable decorative thermoplastic films that are printed in a variety of complex patterns and Figure 2-1 Aircraft interior cabin liner components. Source: FAA (1990).
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft Figure 2-2 Considerations in inclusive design criteria. colors and are embossed in a wide selection of textures and gloss levels. These films are designed to meet rigid airline and manufacturing demands involving color, gloss, and texture. Although these characteristics are largely subjective and therefore difficult to specify and measure, they must nonetheless be consistently matched. Typical panel constructions are shown schematically in Figure 2-3. Fire-resistant textiles have presented some especially difficult problems. Aesthetics demand that upholsteries, draperies, carpets, and tapestries be available in a wide variety of colors and have tight tolerances for look, feel, and durability. New-generation synthetic fibers with improved fire resistance have had a natural dark color that has essentially precluded their being pigmented in light colors, so the lack of availability of a wide variety of colors has greatly limited their acceptance for aircraft application (Hasselbrack, 1995). The major material that is used for upholstery and drapery has been fire-retarded wool, with some use also made of a fire-retarded polyester, both of which meet the fire-resistance requirements and can also be dyed in an unlimited range of colors. Tapestries are held to more stringent flammability requirements than upholstery and drapery. It has been difficult to formulate a fire-retardant scheme for wool that allows it to meet the more-stringent requirements. Therefore, tapestries currently have to be fabricated of the new synthetic materials, with a fairly limited color palette or of wool/synthetic hybrid fabrics. This restriction has, to some extent, discouraged the use of tapestries. Other decorative schemes of less aesthetic appeal are being used in place of tapestries. There are numerous other material types used in various applications. Examples are summarized in Table 2-1. SAFETY The air transport industry and its regulators have achieved an outstanding safety record (as described in Appendix B) by placing an intense, vigorous, and unrelenting priority on the safety of the air transportation system. To maintain and even improve this excellent aviation safety record, the people responsible for the operation and maintenance of aircraft—flight crews, airplane mechanics, and air traffic controllers—are selected and trained according to rigorous safety criteria. Aircraft are designed to operate routinely under extreme conditions, that is, both at altitudes where human life cannot be sustained outside the aircraft and on the ground while taking off and landing at high speeds. The amount of fuel necessary to move such large and highly engineered machines over several thousand miles while at an altitude of seven or eight miles is tremendous. For example, the heat energy contained in the fuel carried by a 747 (more than 50,000 gallons of jet fuel) is more than 20 times as much as would be needed to heat up and melt the entire airframe. Minimum safety standards for aircraft design, manufacture, and operation are established by FAA regulations. In addition to the regulatory mandates, aircraft manufacturers use supplemental design criteria that go beyond the regulatory requirements (for example, see Boeing Commercial Airplane Group, 1977; Airbus Industries, 1979). The criteria for interior safety requirements were developed for normal operation (which includes all non-crash-related incidents) and for several survivable crash-related scenarios (crashworthiness). Although there are many types of criteria, the major ones are: structural strength and stiffness, fire resistance (includes control of smoke generation), interior configuration and emergency evacuation, and emergency oxygen systems. of these, fire resistance structural strength and stiffness have the most impact on research for improved fire- and smoke-resistant materials. Fire Resistance Although there are regulations concerning physical and mechanical properties as well as configuration and layout requirements, the FAA regulatory requirements for interior furnishings are based, in large part, on flammability. The flammability mandates for transport aircraft are listed in FAR 25.853, FAR 25.855, and FAR 25.869. For most furnishings (except cabin liners, seats, and cargo liners) these comprise Bunsen burner tests to characterize resistance to ignition and ability to sustain a flame. In addition to ignitability requirements, cabin liners are subject to additional requirements that
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft Figure 2-3 Typical cabin panel constructions. Provided courtesy of The Boeing Company.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft TABLE 2-1 Materials Applications in Current Commercial Aircraft Applications Materials Floor and floor covering Glass or carbon/epoxy or phenolic/Nomex honeycomb floor panels -flexible urethane seat track covers -urethane foam edge band Mylar film over galley and entry floor panels Wool or nylon carpet -double-backed tapes to attach carpet to floor -Nomex felt underlay (at customer request) Poly( vinyl chloride) galley mats Lower sidewall panel Glass or carbon/phenolic/Nomex honeycomb plus scuff resistant surface (wool or Nomex fabric, or tough plastic) Upper sidewall panel Glass or carbon/phenolic/decorative thermoplastic layer plus Tedlar Light covers Polycarbonate Overhead stowage bins Glass or carbon/phenolic/Nomex honeycomb plus edge urethane foam layer plus reinforcement Gap fillers Silicone or urethane Passenger and cabin attendant seats Wool, wool/nylon, or leather upholstery Urethane foam cushions Polybenzimidazole or Nomex/Kevlar blocking layer Polyethylene form flotation foam Thermoplastic seat trays Partitions Glass or carbon/phenolic/Nomex honeycomb Decorative thermoplastic laminate or wool/Nomex textile or leather Polycarbonate transparent wind screen (infrequent) Stowage bins Glass or carbon/phenolic/Nomex honeycomb Decorative thermoplastic laminate Wool textile interior liner (infrequent) Placards Poly(vinyl chloride) or urethane Insulation Fiberglass batt, phenolic binder, Mylar cover Poly(vinyl chloride)/nitrile rubber, polyethylene, foams Polyimide foam Windows Outer pane stretched acrylic Inner pane cast acrylic Dust cover polycarbonate or acrylic Passenger service units Molded thermoplastics (Ultem, Radel, polyethelketoneketone) Aluminum Glass or carbon/phenolic Hoses Silicone Nylon Urethane Air ducts Glass/phenolic, epoxy, or polyester for large ducts Polyisocyanurate foam for large ducts Fire-retarded nylon Glass/silicone Nomex felt (small quantity) Polyimide foam wrap Source: NRC (1995).
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft TABLE 2-2 FAA Flammability Requirements for Cabin Liners (sidewalls, ceilings, and partitions) Test Type Current Minimum Acceptance Criteria Ignitability (60-second vertical Bunsen burner) 6-inch bum lengtha 15-second specimen extinguishing timeb 3-second drip extinguishing timec Heat release (Ohio State University calorimeter) 65 kW/m2 peak rate (during a 4-minute test). d 65 kW • min/m2 total (during the first 2 minutes)e Smoke release (National Bureau of Standards smoke chamber) 200 specific optical density (during a 4-minute test)f NOTE: Definitions and test procedures are described in detail in FAA (1990). a Burn length: the distance from the original specimen edge to the farthest evidence of damage to the test specimen due to that area's combustion. b Specimen extinguishing time: the time that the specimen continues to flame after the burner flame is removed from beneath the specimen. c Drip extinguishing time: the time that any flaming material continues to flame after falling from the specimen to the floor of the test chamber. d Heat release rate: the rate at which heat energy is evolved by a material when burned. The maximum heat release rate occurs when the material is burning most intensely. e Heat release: a measure of the amount of energy evolved by a material when burned. f Specific optical density: a dimensionless measure of the amount of smoke produced per unit area based on light transmittance measurements. involve control of total heat release and heat release rate and density of smoke produced. Seats and cargo liners must meet rather severe tests based on kerosene oil burners of the sort used in home heating furnaces. Detailed descriptions of flammability test methods for individual aircraft components are described in "The Materials Fire Testing Handbook" (FAA, 1990). Table 2-2 summarizes the flammability requirements for cabin liners. Current regulations covering the flammability of interior cabin furnishings apply directly to individual parts that make up the interior, for example, those comprising sidewalls, ceiling, partitions, stowage bins, windows, air ducts, and insulation. There is a multitude of such parts; for example, sidewalls consist not just of multiple copies of certain parts, but of a large number of parts with different part numbers. Other components also make up long lists of part numbers. Many of the current fire-related regulations were based largely on recommendations of the FAA SAFER committee (FAA, 1980). The SAFER committee focused on ways to make post-crash fires more survivable. Their findings concerned fire hazards associated with spilled fuel ignition and burning, hull burnthrough, and involvement of cabin materials and escape slides. The recommendations to the FAA that were associated with fire-resistant materials included: establishing contribution of cabin materials to fire hazard based on large-scale tests; developing seat fire-blocking layers; expediting the development and evaluation of the Ohio State University (OSU) heat-release test chamber; accelerating toxicity research; amending the flammability test methods to account for melting and drip behavior of certain materials; defining fire scenarios for modeling, research, and design; validating models with small-and full-scale tests; establishing radiant heat-resistance standards for inflatable evacuation devices; expediting the development of fire-resistant cabin windows; and supporting development of fire-resistant cabin interior materials and encourage the development of a materials data bank. Action based on many of the SAFER committee recommendations have contributed to the development of current flammability standards, including seat fire-blocking regulations, cabin liner materials heat release regulations using the OSU test, and a Technical Standard Order requiring radiant heat resistance for escape slides. Strength and Stiffness Requirements The design criteria include strength and stiffness requirements. All cabin interior components must be designed to routinely withstand "limit loads" (i.e., typical flight loads).
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft For a margin of safety, components must also be able to withstand occasional "ultimate loads" (1.5 times limit loads) and "assist loads" or "abuse loads" (e.g., bumping, pushing, and pulling handles). Components are designed to undergo only elastic deflections under these conditions. In addition to the loads cited above, some components such as seats, stowage bins, closets, and class dividers must also be able to safely restrain items of mass (e.g., passengers or stowed items) under "minor" crash loads, as specified in FAR 25.561: The airplane, although it may be damaged in emergency landing conditions on land or water, must be designed as prescribed in this section to protect each occupant under those conditions. The structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing when— Proper use is made of seat belts and other safety design provisions; The wheels are retracted (where applicable); and The occupant experiences the following inertia forces acting separately relative to the surrounding structure: Upward, 3.0g Forward, 9.0g Sideward, 3.0g on the airframe; 4.0g on the seats and their attachments Downward, 6.0g Rearward, 1.5g The supporting structure must be designed to restrain, under all loads up to those specified in paragraph (b) (3) of this section, each item of mass that could injure an occupant if it came loose in a minor crash landing. Seats and items of mass (and their supporting structure) must not deform under any loads up to those specified in paragraph (b) (3) of this section in any manner that would impede subsequent rapid evacuation of occupants. Furthermore, the seats and seat/seat track attach points must meet dynamic load conditions that underlie criteria for restraint loads and head injury, leg injury, and seat deformation. From the above mandates on the components themselves, the designer must evaluate the loads the constituent materials must sustain and what the measurable physical and mechanical properties need to be. Typical mechanical properties include tensile strength and stiffness; compression strength and stiffness; shear strength and stiffness; flexural strength and stiffness; impact strength; and pull-out, torque, and shear strengths of insert installations. To establish mechanical property limits used in design, materials (especially those used in parts that must bear structural loads) are thoroughly tested at various representative temperatures and environments that the material might experience during its service lifetime. Beyond this, however, the certification of components that would potentially bear critical loads requires that each newly designed component be structurally substantiated by comparison for similarity to previously certified components, analysis, or test. Components are designed to absorb crash loads through component deformation (yield). For example, controlled deformation of seat structure is the primary means through which designers have been able to meet the requirements for dynamic loading of seats and seat track attachments. If the seat were totally rigid and all the crash energy transmitted directly to the passenger, the passenger may not survive the crushing effect of a safety belt. WEIGHT Weight is one of the most important criteria in designing an aircraft. The economic viability of an aircraft's operation puts a priority on cost, payload, and range, which are directly impacted by weight. Airplane designers are keenly aware of the value of a pound of weight, which is a cost derived from the fact that if the empty airframe weight is reduced by a pound, not only is less fuel needed for the airplane's operation for a given range, but also more payload and fuel (more fuel equals more range) can be carried. Over the life span of an airplane in service (typically more than 20 years), the additional fuel needed to transport an extra pound on the airframe amounts to $200–$400 at current fuel prices ($0.60–$0.80 per gallon at 6.66 pounds per gallon) . 1 The cost of losing payload or range is more difficult to establish, but nevertheless, it puts the economic viability of the aircraft at risk. COST Cost is another critical criterion in designing an aircraft. The cost of procuring and maintaining an aircraft interior 1 An average wide-body aircraft on a 3,000-mile flight might expend 0.25 pounds of fuel for every additional pound to the interior. Considering 10 such flights per week for a year and current fuel costs ($0.60 per gallon), the aircraft's operating cost would increase by $12 per year per additional pound. For a narrow-body jet, assuming 1,200-mile flights and 24 flights per week, the additional fuel cost would increase by $20 per year per additional pound.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft factors into the total cost of running an airline and must be taken into account in interior design and manufacture. Technological advances must "earn" their way into implementation by means of either resultant decreases in operating costs or customer-approved improvements in operation that justify increased costs. Since all airlines, for marketing reasons, strive to present a specific and identifiable image, the interior furnishings tend to have a unique decor and are therefore in a sense custom made, which results in higher costs. The costs of such detailed design and manufacture must therefore be amortized over fewer units of aircraft than many mass-produced commodities. There is a priority, therefore, to optimize the efficiency of all processes involved to provide cost reduction. The manufacturing costs for components are composed of materials costs and processing costs. Reducing each of these has a priority, but the installed part cost is decisive. In this way, an expensive material that is easily and inexpensively processed may be favored over a less expensive material that is difficult to process.2 MANUFACTURABILITY AND PRODUCIBILITY The need to reduce production costs has put great pressures on aircraft designers, including cabin interior designers, to create designs with a priority on the ease of manufacturing and assembly. One method that has produced good results involves teams that include representatives from engineering, quality assurance, and manufacturing working together to create designs that can be manufactured more cost effectively. A component's form, fit, function, quantity required, and other design criteria are combined with the various possibilities of materials and processes (namely, materials cost, processing labor, and capital equipment and tools) to define the final part. Standard parts that are interchangeable among all customers (e.g., air-conditioning ducts and cargo liners) need to be designed with materials and processes that minimize processing labor since they can be produced in higher volumes. Customized parts, such as cabin liners that have a common construction for all customer airlines but a customized texture and decor, need to be designed with materials and processes that allow rapid change and flexibility. Processing Labor As a general rule, processing labor accounts for most of the cost of a pan. However, there is a widespread range of processing costs that depend on the materials and manufacturing processes selected. Factors that must be considered when designing nonmetallic parts include: part quantity; capability for integration of multiple details; texturability; control of surface smoothness; thickness consistency; edge close-outs; bonding; machining and drilling; trim and finish; conditioning; environment and temperature compatibility when coforming dissimilar materials; storage, handling, and shelf life; compatibility with fasteners and inserts; part size; and interchangeability. Examples of processes commonly available for the manufacture of aircraft interior parts include composite lamination and press curing, compression molding, hydroforming, casting, pultrusion, and filament winding of thermosetting polymers and composites and blow molding, rotational molding, thermoforming, injection molding, casting, and extrusion molding of thermoplastic polymers and composites. The most common processes used in the fabrication of components for current aircraft cabins are composite lamination followed by press curing. Honeycomb core with fiber-reinforced phenolic face sheets make up the majority of cabin interior panels (Berg, 1995). Common thermoplastic processes currently used include sheet thermoforming and injection molding. The amount of processing labor depends on the part configuration and the process used. For example, a part constructed from a fiber-reinforced composite laid up by hand takes considerably more time to prepare, lay up, cure, trim, and finish compared to the time required to process an injection-molded part. It is therefore important that the design of a part take account of the manufacturing process that has to be used to produce the design and that the materials and processes are jointly optimized to minimize manufacturing cost. 2 In general, the procurement cost to the airline of a component, and the influence of materials and processing costs, is similar whether the component is part of the delivered equipment or installed subsequently. In the case of a retrofit or refurbishment, the added cost to the airline would be associated with processes such as removal and disposal of the old component, development of plans and procedures for maintenance crews to follow, airplane downtime, etc. These costs have more to do with regulatory mandates that may compel upgrades of the existing fleet than with materials development and selection decisions and priorities.
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft Processing Equipment and Tools A part's design and manufacturing process affects the tooling required as well as the processing labor. During component design, designers need to ensure that the manufacturing tooling required is compatible with the part's size, form, and the manufacturing process. The quantity of parts to be produced also affects the material and process selection. For example, injection molding is appropriate only for production of large quantities of parts because injection-molding tooling is expensive. COMFORT Materials properties, component design, and the interior layout (configuration) all contribute to passengers' convenience and comfort. Important comfort factors for this study are seat ergonomics and acoustics. Seat Ergonomics Aircraft manufacturers do not design or produce passenger seats. Although aircraft manufacturers provide standard seat specifications to ensure correct fit and adherence to safety and certification requirement, passenger seats are manufactured by companies that specialize in them. Generally, airlines work directly with seat manufacturers to specify and purchase passenger seats. The seats are then manufactured to the Figure 2-4 Typical seat construction. Source: FAA (1990). airline's specifications, which must of course include provision for appropriate means for attachment to the seat tracks on the cabin floor that are provided by the aircraft manufacturer. Noisily, the seats are then shipped to the aircraft manufacturer for installation in the cabin prior to delivery of the airplane, although in some instances the seats are shipped tot he Customer airline, which takes delivery of the airplane and subsequently installs the seats at its own maintenance facilities. Typical seat construction is shown in Figure 2-4. In the 1970s it was recognized that the greatest fire load in the passenger cabin was by far the seat cushion foams which were exclusively fire-retarded flexible polyurethane. In 1986 the regulatory requirements were changed to require that the upholstery/cushion combination satisfy a severe fire-resistance test. This mandate essentially required that the seat cushions be changed from the traditional fire-retarded urethane foam. The first available solution was to encapsulate the urethane foam in a durable fire-blocking fabric. Subsequently very highly fire-retarded foams (basically filled urethanes) were developed that also met FAA requirements. Alternatives to urethane foams for seat cushions have not been successfully developed. The upholstery used on seats is typically a highly durable woven fabric of wool or a wool/nylon blend (the amount of nylon is relatively small, with 10 percent being the usual amount). While a type of fire-retarded polyester and leather have also been used as seat upholstery, no other natural or synthetic fiber type has been successfully implemented because of limitations with respect to fire resistance, color, durability, availability, color fastness, and other criteria (Hasselbrack, 1995). Acoustics Interior acoustics comprise a significant comfort factor. The major portion of cabin noise is generated by the engines and by the aerodynamic boundary layer along the exterior surface of the fuselage. Noise is reduced through absorption and transmission-loss mechanisms. Higher frequencies (above 500 Hz) are more easily absorbed, whereas lower frequencies must be blocked with mass (transmission loss). Acoustic insulation is provided by the cabin insulation blankets, which also serve as thermal insulation. The thermal/acoustical insulation is comprised primarily of spun fiberglass that has a binder to hold the strands together. Owing to the acoustic requirements, the fiber diameter is very much smaller than the fiber diameter of typical building insulation. Also, there have been foams, typically of polyimide, that have been used in recent years to supplement the fiberglass. The insulation material is encased in a thin film to hold it in place. The film is typically polyethylene terephthalate (Mylar®), although in the past polyvinylfluoride (Tedlar®) and polyimide (Kapton®) have also been used. Kapton films
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft have a substantial weight and cost penalty compared to Mylar. Insulation blankets, used in areas where low-frequency sound must be blocked (e.g., near engines), are encapsulated in a heavy rubberized sheet material, such as a zirconium-filled silicone rubber. Cabin linings also help to absorb some of the higher frequencies, but more importantly block some of the lower frequencies. Once in the interior, high-frequency cabin noise is reduced by using porous cloth upholsteries, carpets, and draperies. The sound penetrates these materials and becomes absorbed. Sound waves more easily bounce off leather upholsteries. The system of the insulation blankets, cabin linings, and furnishings combine to reduce exterior-generated sound by as much as 80–130 decibels. In acoustic design, there is a desirable limited level of background noise in the cabin to provide a measure of privacy. However, as the noise level rises above this background level, it at first becomes difficult to carry on a conversation with a person close by, and then the noise level becomes physically discomforting. Acoustic designers pay attention to the "speech interference level," that is, the 1,000-, 2,000-, and 4,000-Hz bands, to avoid undue interference with speech. In the event of a post-crash fuel-fed fire, the fiberglass insulation (if it remains attached) also provides a degree of protection as a fire barrier to inhibit the spread of fire to the cabin interior. The polyimide foams can also serve as such a barrier. MAINTAINABILITY Maintenance activities represent a significant part of the time and expense related to the operation of commercial aircraft. A cost-effective maintenance program is critical to the implementation of new materials or component designs. Maintainability has to do with the ease of preserving the passenger cabin interior in a satisfactory and usable condition. For interior furnishings, maintainability depends on component cleanability, durability, and repairability. Cleanability The major part of maintainability is the ease with which furnishings can be cleaned of normal soiling. Normal soiling agents consist of many things that are typically found in ordinary airline operations. Those that are difficult to remove include some types of dirt and mud; food stains such as mustard, catsup, grape juice, and red wine; cosmetics such as lipstick; and tobacco tar from cigarette smoke. Traditionally, candidate interior furnishings have been tested for cleanability before being implemented (i.e., for how easily common staining agents can be removed). Materials that are highly resistant to abuse such as scuffing and scratching, to color degradation by agents such as ultraviolet or even visible light, and to staining by common staining agents are required. In general, a thin film of polyvinylfluoride or equivalent has been found to be nearly optimum with respect to these in-service requirements. Less desirable are paints from which stains are more difficult to remove (repainting is often necessary) and which are more easily degraded. Flooring in some areas such as galleys, entry ways, and lavatories must be water resistant and skid resistant and easily cleanable for sanitary as well as aesthetic reasons. Durability Durability is how well components wear over time in everyday airline operation. To be economically viable to an airline's operation, an airplane—including its interior—must be durable. Durability factors and requirements important for aircraft interiors include: resistance to vibration, colorfastness to light, resistance to tearing, resistance to crocking (textiles), dimensional stability, fluid resistance (e.g., solvents and cleaning fluids), resistance to permanent staining, resistance to water and moisture absorption, resistance to water wicking, resistance to corrosion, resistance to fungus attack, resistance to abrasion damage, resistance to impact damage, and resistance to crazing. Many of these factors are obvious. For example, cabin liners. class dividers, and other exposed surfaces need to be reasonably resistant to mild abuse, such as the inadvertent contact with ends of stick-shaped items such as umbrellas. Also, stowage bins need to be quite tolerant of, for example, attempts to force in items that are too large or passengers who cannot quite reach up to the bin hanging onto the bin bottom. It is not difficult to envision other examples. Other factors have more subtle implications. Thermal/acoustical insulation and wiring materials, for example, must be resistant to vibration damage. Poor thermal/acoustical insulation batting may be subject to crumbling and settling which would reduce the insulation efficiency, and poor wire
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Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft insulation may be prone to chafing which could increase the risk of electrical arcing and fire. Because of the extensive use of aluminum in current aircraft, it is necessary to be concerned with galvanic corrosion. Thermal/acoustical insulation also needs to be resistant to wicking and water absorption to avoid promoting corrosion. Textiles can cause corrosion problems due to the fire-retardant treatments involving flame-inhibiting salts that may promote corrosion if brought into contact with metal surfaces. Fabric treatments need to be tested to ensure they do not promote corrosion. Corrosion resistance is also pertinent to some of the newer composites that use reinforcement fabrics of carbon and graphite, which have galvanic potentials considerably different from those of metals, especially aluminum. Special sealing requirements are required to electrically isolate these composite materials from aluminum components such as seat tracks, fasteners, and floor beams. Repairability Even though interior furnishings may be durable, there is inevitable damage that occurs which requires that the affected item either be repaired or replaced. In general, it is substantially less costly to repair damage to an interior furnishing than to discard and replace it. Although a damaged furnishing may be removed and replaced to allow the airplane to continue in uninterrupted service, the airline normally pursues repair of the item and stores it as a spare. Therefore there is a priority for repairability of reasonable damage to interior furnishings and to have repair processes and procedures identified and documented by the manufacturer. AESTHETICS Airlines use an airplane's interior decor, and to some extent the exterior decor, as a marketing tool. This is intended to reflect the airline's corporate image, style, and mission. Aircraft interior designers therefore need to have a variety of colors, prints, and textures and different degrees of plushness for airlines to select from to build a unique image and identity. For newly manufactured aircraft, the basic shape and architecture of the interior are normally the standard ones offered by the aircraft manufacturer. The choices the airline customer has are mainly centered on decorative schemes to reflect the corporate image and perhaps the placement of some amenities such as lavatories and galleys. Interiors for newly manufactured aircraft are normally designed by the aircraft manufacturer in coordination with the customer airline, but their actual fabrication is done by various combinations of the aircraft manufacturers and subcontractors who specialize in the manufacture of interior furnishings. With proper maintenance, interiors are designed to last as long as the aircraft. Airlines often update the interiors of aircraft in their fleet to present a "new look." Interiors may also be refurbished or remodeled at some time during service due to transfer of the aircraft between airlines or safety-related retrofit. These interior furnishings are for the most part also fabricated by manufacturers who specialize in the manufacture of interior furnishings. In both newly manufactured and redecorated aircraft, different levels of decor and amenities are used to target different segments of the passenger population. Business and first class travelers expect and are usually provided a higher level of comfort and luxury than passengers in the economy class; however, owing to the competition between airlines in the pursuit of passengers, the interior furnishings for all classes of service receive considerable attention. The types of routes an airline flies and the airports it serves also affect aesthetic considerations. For example, airlines that fly into airports serviced by airstairs have their cabin interiors substantially affected by weather. Hot tarmac particles in the summer, snow in the winter, and combinations of rain, wind, and dirt during all seasons are constant problems. Another consideration is that airlines that provide international service to different areas of the world must contend with aircraft serviceability and maintenance requirements at locations far removed from their home base.