Materials with reduced flammability in aerospace and aviation

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Materials with reduced flammability in aerospace and aviation R E L Y O N , Federal Aviation Administration, USA

Abstract: This chapter reviews the history of aircraft fire regulations, the types of materials used in transport category aircraft, their location in the fuselage, and the test methods and requirements for their flammability. Key words: aircraft, fire, flammability, plastics, regulations.

20.1

Introduction

Fire safety is designed into transport aircraft in order to prevent in-flight fires and mitigate post-crash fires, which account for about 20% of the fatalities resulting from airplane accidents.1,2 Before an airplane can carry passengers, the manufacturer must satisfy airworthiness criteria3 established by the regulatory authorities that certify the airplane design. In addition to the certification requirements, manufacturers may impose supplementary safety criteria that go beyond the regulatory requirements.4 The regulatory authorities will issue a type certificate for a particular design (e.g., Boeing 747-100 Series) if the design complies with the regulations, including the fire-worthiness (flammability) requirements for all of the parts used to construct the airframe, power plants, and cabin interior. An airplane having new or novel materials or designs not envisioned when the regulations were promulgated can be certified under Special Conditions if the manufacturer demonstrates a level of safety equivalent to the existing certification requirements. Once the airplane is certified, changes in the construction of cabin materials and components can only be made with the approval of the regulatory authorities and only if the materials/components meet the minimum fire performance standards for the design. The Federal Aviation Administration (FAA) in the United States and its European counterpart, the European Aviation Safety Agency (EASA) are two of the regulatory authorities that certify the airworthiness of aircraft in addition to the individual countries of registry. Manufacturers of transport category (>20 passenger) aircraft in service worldwide include Boeing, Airbus Industries, Embraer, and Bombardier. Bilateral Aviation Safety Agreements are used to facilitate approvals between countries so that aircraft can be certified and sold outside of their country of manufacture. Once the airplane is manufactured, certificated, and sold to airlines, it is the responsibility of the owner airlines to assure the safe operation of their airplanes

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in accordance with flight and maintenance standards imposed and monitored by the local regulatory authorities (e.g., FAA, EASA) in cooperation with the International Civil Aviation Organization (ICAO) and the aviation authorities of Canada, Brazil, Israel, China and Russia. These aviation authorities ensure continued airworthiness through working arrangements aimed at harmonizing standards and promoting best practices in aviation safety worldwide. Mandatory changes to materials and components of in-service aircraft may be required if an unsafe condition is identified by the regulatory authorities. In this case, the regulatory authority issues an Airworthiness Directive that mandates a fire safety upgrade or replacement according to a prescribed schedule. Materials and components of in-service aircraft can only be changed voluntarily if the operator or manufacturer demonstrates an equivalent level of safety to the certificated design. As with the manufacturers, the airline operators often impose additional internal safety criteria that go beyond the regulatory requirements.4 The following sections describe the history of aircraft fire regulations, the types of materials used in transport category aircraft and their location in the fuselage, as well as the test methods and requirements for the fire-worthiness of aircraft cabin materials.

20.2

History of aircraft fire regulations

Aircraft fire regulations cover many aspects of aircraft as well as many different kinds of aircraft including small airplanes, large airplanes, airplanes that combine passenger and cargo spaces, helicopters, etc. In the United States the rules governing Aeronautics and Space are codified in Title 14 of the Code of Federal Regulations (14 CFR).3 Parts 1 through 59 of Chapter 1, 14 CFR, define the rules and responsibilities of the US aviation regulatory authority, the Federal Aviation Administration (FAA). The 14 CFR Parts 1±59 are commonly referred to as the Federal Aviation Regulations (FARs). The Airworthiness Standards for Cargo and Commuter Category Airplanes are the subject of 14 CFR Part 23, while the Airworthiness Standards for Transport Category Airplanes are the subject of 14 CFR Part 25. The paragraphs in 14 CFR Part 25 that govern the flammability of materials in transport category aircraft are Paragraphs 853 (Compartment Interiors), 855 (Cargo or Baggage Compartments), 856 (Thermal/ Acoustic Insulation), and 869 (Electrical Wiring), and these will be referred to in the sections that follow as FAR Part 25.853 or Section (§) 25.853, etc. Further definition, test criteria, and guidance for showing compliance with § 25.853, § 25.855, § 25.856, and § 25.869 are given in Appendix F of 14 CFR 25 and the Materials Fire Test Handbook,5 which is updated periodically by the FAA to reflect best practices. These fire regulations are derived from research and development conducted principally by the Federal Aviation Administration (FAA) in anticipation of, or in response to, aircraft incidents and accidents.2,6 Since the US fire standards are used internationally, and typically adopted by

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reference, the discussion of fire regulations will focus on the history of the FARs as outlined by Peterson.7 In 1945 the first regulatory fire performance requirements covering the certification of transport category aircraft design in the United States were introduced as Civil Air Regulation 4b (CAR 4b) by the Civil Aeronautics Agency (CAA). The first jet transports manufactured by Boeing (707, 727) and McDonnell Douglas (DC-8, DC-9) were certified to CAR 4b. In 1965, the US Department of Transportation (DOT) was created and the CAA was reorganized and made a part of the DOT as the Federal Aviation Administration (FAA). Simultaneously, the existing CARs were reissued without change as parts of the FAR, with CAR 4b becoming FAR Part 25. The FAA has periodically introduced new regulatory requirements through amendments to FAR Part 25 or Special Conditions. Several of these have been issued that have imposed more stringent flammability requirements on interior cabin furnishings. These Part 25 amendments are effective only for new designs and do not directly affect airplanes being manufactured to existing design criteria. The mechanism used by the FAA to mandate changes to existing and/or newly manufactured airplanes built to existing designs is to amend FAR Part 121, which covers what requirements an airplane must meet, in addition to its Part 25 certification basis, before the airlines can operate it in passenger service. Included in the requirements of CAR 4b and FAR Part 25 Original Issue were that for each compartment to be used by the crew or passengers the materials must be at least `flash-resistant,' which meant that the construction, when tested horizontally using a Bunsen burner with a 15-second ignition time, will have a burn rate less than 508 mm (20 in.) per minute. Large area materials including wall and ceiling linings and the covering of upholstery, floors, and furnishings were held to a higher standard of `flame-resistant,' which reduced the maximum burn rate to 102 mm (4 in.) per minute in the 15second horizontal Bunsen burner test. Receptacles for towels, paper or waste were required to be `fire-resistant,' which meant that the construction, when tested at a 45 degree angle with respect to horizontal using a Bunsen burner with a 30-second ignition time, allows no flame penetration, self-extinguishes in less than 15 seconds, and glows for less than 10 seconds after removal of the Bunsen burner flame. In 1967 the FAA introduced Amendment 15 to FAR Part 25 (FAR 25-15) to reduce the ignitability of aircraft materials. Amendment 25-15 imposed the more stringent requirement of a vertical Bunsen burner test having a 12second ignition time and introduced Appendix F, which is a description of most of the test procedures used to demonstrate compliance with the flammability requirements in FAR Part 25. The Amendment 25-15 vertical Bunsen burner test applied to interior wall and ceiling panels, draperies, structural flooring, baggage-racks, partitions, thermal insulation and coated fabric insulation covering. Amendment 25-15 included a char length of 20 cm

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(8 in.) for the 12-second vertical Bunsen burner test and modified the acceptance criteria for the horizontal Bunsen burner test to include a maximum char length of 10 cm (4 in.). The FAR Part 121 was not amended to require any changes to existing or newly manufactured aircraft when FAR Part 25-15 was introduced. In 1972, the FAA introduced Amendment 32 to FAR Part 25 (Amendment 25-32), which was developed by the Aerospace Industries Association (AIA) in their 1966±1967 Crashworthiness Program. FAR Part 25-32 superseded FAR 25-15 for new designs and is still required for certification of new designs (see Section 20.4, Fire Test Methods for Aircraft Materials). Amendment 32 added a requirement for electrical wire insulation that was a 30-second, 60-degree Bunsen burner test and upgraded the requirement for large area interior material (ceiling and wall panels, stowage compartment materials, partitions, galley structure, large cabinet walls, and structural flooring) to a 60-second vertical Bunsen burner test (see 60VBB for performance criteria). The 12-second vertical Bunsen burner test of FAR 25-15, modified to include flame times of sample and dripping, was retained for large area materials not specifically covered by the new 60-second rule, e.g. floor covering, textiles, cargo liners, seat trays, air ducting, insulation blankets, transparencies, thermoformed parts, electrical conduit. All materials not covered by the vertical Bunsen burner test with 12- or 60-second ignition were tested in the 15-second horizontal Bunsen burner test of the 1965 FAR Part 25 Original Issue. The Boeing 747 airplane was certified in 1969 to special conditions equivalent to FAR 25-32, while the McDonnell Douglas DC-10 and Lockheed L-1011 were certified to FAR Part 25-32 in 1972. FAR Part 121 was not amended to require any changes in existing or newly manufactured aircraft when FAR Part 25-32 went into effect. In the early 1970s the FAA initiated proposed rulemaking activities that involved smoke and toxic gas emissions of burning cabin materials that would amend FAR Parts 25 and 121. Although these proposals were withdrawn in 1978 in favor of a more systematic approach recommended by the Special Aviation Fire and Explosion Reduction (SAFER) committee, aircraft manufacturers had already developed internal acceptance criteria for smoke and toxic gas emission of large area cabin materials. These manufacturers' voluntary criteria were intended to provide guidance for material suppliers in the development of new cabin materials and are still in effect. These internal guidelines vary by manufacturer but go beyond those proposed by the FAA. Boeing criteria varied with the type of airplane. Airbus Industries unilaterally established requirements in 19798 that were similar to the Boeing guidelines. The McDonnell Douglas Aircraft Company imposed smoke and toxic gas emission criteria that varied depending on the application of the materials. In 1984, the FAA introduced Amendment 59 to FAR Part 25 (FAR 25-59) to reduce the extent that seat cushions, usually made of polyurethane foam, would become involved in a cabin fire. Amendment 25-59 added a requirement to FAR

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25 that seat cushions, except those on flight crewmember seats, must pass an oil burner test. FAR Part 121 was amended (FAR 121-184) to require retrofitting of all passenger seats to comply with FAR 25-59 by 1987. In 1986, the FAA introduced Amendment 60 to FAR Part 25 (FAR 25-60) to contain cargo fires within cargo compartments by imposing an oil burner test for flame penetration of Class C and D compartment liners. Also in 1986, the FAA introduced Amendment 61 to FAR Part 25 (FAR 2561) to reduce the flammability of passenger cabin sidewalls, ceilings and large area cabin linings in a post-crash, fuel-fed fire in order to increase the time available for passengers to escape from a burning airplane. Amendment 25-61 added a heat release requirement to FAR 25 for interior ceiling and wall panels, partitions, and the outer surfaces of galleys, large cabinets and stowage compartments. FAR Part 121 was amended (FAR 121-189) simultaneously with Amendment 25-61 and required that all aircraft manufactured after 20 August 1990 comply with Amendment 25-61. In 1988, the FAA introduced Amendment 66 to FAR Part 25 (Amendment 25-66) that clarified the heat release requirements of FAR 25-61 and added a smoke requirement to those materials that would reduce the smoke emitted in a post-crash fire. FAR Part 121 was amended (FAR 121-198) simultaneously with FAR 25-66 and required that all aircraft manufactured after 20 August 1990 comply with FAR 25-66. In 1991, the FAA introduced Amendment 202 to FAR Part 121 (FAR 121202) and Amendment 31 to Part 135 (FAR 135-31) to require an oil burner flame penetration test, as per FAR 25-60, for Class C and D cargo liners that were not constructed of fiber glass or aluminium. In 2003, the FAA introduced Amendment 111 to FAR Part 25 (FAR 25-111) to add new fire performance requirements for flame spread and fire penetration of thermal/acoustic insulation. FAR Parts 91, 121, 125, and 135 were amended simultaneously to reflect the new requirements. Newly manufactured airplanes and material replacements on in-service airplanes were required to comply with the flame-spread requirement by 2 September 2005. Newly manufactured airplanes are required to meet the fire penetration (burn through) requirement by 2 September 2009. Figure 20.1 is a timeline of FAA regulatory upgrades for materials flammability.

20.3

Materials used in commercial aircraft

Interior cabin furnishings must meet the basic production and functional requirements of the aircraft manufacturers as well as the serviceability and aesthetic requirements of the airline operators.4 They must also meet the fire safety requirements specified in the airplane certification process.3 Materials used in transport category aircraft are roughly the same regardless of the type of aircraft involved. The various Federal Aviation Regulations (FARs) refer mostly

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20.1 Timeline of aircraft flammability regulations.

to FAR Part 25 for flammability requirements. For simplicity this discussion will refer only to FAR Part 25. The combustible cabin materials covered by FAR Part 25 weigh about 3000 kilograms for a narrow body aircraft to over 7000 kilograms for a wide body aircraft as shown in Table 20.1.9 The distribution of weight by major classes of materials for a wide body aircraft is shown in Fig. 20.2. Since the heat of combustion of these materials is roughly similar at about 20 MJ/kg, Fig. 20.2 also represents the distribution of fire load. The total fire load represented by the mass of materials in Table 20.1 is of the order (7600 kg) (20 MJ/kg) = 152 000 MJ, which approximates the heat of combustion of 1000 gallons of jet fuel, or about 2% percent of the fuel capacity of a wide body aircraft. The construction of major aircraft cabin components below is an update of ref. 5 and the breakdown of materials by application in Table 20.2 is an update of ref. 10.

Table 20.1 Combustible materials in wide body commercial aircraft9 Component Seats Acoustic insulation Decorative panels Textiles Air ducts

Weight (kg) 1500 400 1600 900 500

Component Linings Electrical insulation Windows Small parts & rubber Safety equipment

Weight (kg) 500 200 500 500 1000

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20.2 Distribution of combustible materials in wide body transport category aircraft by weight.

Aircraft seats Aircraft seats are made from a wide variety of nonmetallic materials. These components can be grouped into five basic areas: foam rubber cushions, upholsteries, fire blocking layer, plastic moldings, and structure. Small nonmetallic seat parts must meet FAR 25.853(b). A pending special condition for large area components is that they also meet the heat release and smoke requirements of FAR 25.853(a) and (a-1) as described in Appendix F Parts IV and V. The cushions, which include the foam rubber, upholstery fabric, and (optionally), a fire blocker, must also meet FAR 25.853(c). Aircraft seats include a wide variety of plastic moldings for items such as decorative closeouts, trim strips, food trays, telecommunication (audio/ video/phone) devices, and arm rests. Heat resistant or flame retardant thermoplastics (polycarbonate, polyvinylchloride/acrylic blends, etc.) are commonly used for these purposes. Most seat structures are made of aluminium; however, some manufacturers have introduced carbon composite structures to reduce weight and light metal alloy frames are also being considered. Thermal-acoustical insulating materials The entire pressurized section of the aircraft is generally lined with thermal/ acoustical insulation, which is by far the largest volume (but not weight) of

Table 20.2 Materials use by application in large aircraft cabins Application

Material

Floor and floor covering

· Glass/carbon-reinforced epoxy/phenolic over aramid fibre honeycomb with flexible urethane seat track covers and urethane foam edge band · Mylar film over galley and entry floor panels · Wool or nylon carpet attached to floor by double backed tape with optional aramid felt underlay · PVC galley mats Glass/carbon-reinforced phenolic with decorative thermoplastic laminated film having PVF outer layer Glass/carbon-reinforced phenolic with decorative thermoplastic laminated film having PVF outer layer

Lower sidewall panel Upper sidewall panel Light covers Overhead storage bins Gap fillers Passenger seats

Cabin attendant seats Partitions Stowage bins Placards Insulation Windows Passenger service units Hoses Air ducting

Polycarbonate Glass/carbon-reinforced phenolic with decorative thermoplastic laminated surface film having PVF outer layer, urethane foam edge Silicone or urethane · Wool, wool/nylon, or leather upholstery · Urethane foam cushions · Polybenzimidazole or aramid fibre fire blocking layer · Polyurethane flotation foam · Thermoplastic seat trays and telecommunication equipment · Wool, wool/nylon, or leather upholstery · Urethane foam cushions · Polybenzimidazole or aramid fibre fire-blocking layer · Polyethylene flotation foam Glass/carbon phenolic with decorative thermoplastic laminated film with PVF outer layer or wool/aramid fibre textile or leather Glass/carbon phenolic surfaced with decorative thermoplastic laminated film with PVF outer layer, wool textile interior liner (infrequent) Polyvinylchloride or polyurethane Fibre-glass batting with phenolic binder and PVF or polyimide cover, or foamed polyvinylchloride, nitrile rubber, polyethylene, or polyimide. · Outer pane stretched acrylic (polymethylmethacrylate) · Inner pane cast acrylic · Dust cover polycarbonate or acrylic Molded heat resistant thermoplastics (PEI, PPSU, PEKK) or aluminium or glass/carbon reinforced phenolic Silicone, nylon, or urethane · Glass-reinforced phenolic, epoxy, cyanate ester, or polyester over poly(isocyanurate) foam (large ducts) · Fire retarded nylon, glass-reinforced silicone, aramid fibre felt over polyimide foam · Fiber-reinforced polyphenylsulfone and polyetherimide

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nonmetallic material in an aircraft. The acoustical requirements for the insulation are more demanding than the thermal requirements, and govern the amount of insulation used. The insulation blanket construction typically consists of a low-density glass-fiber batting surrounded by a protective cover. In some applications, where the insulating material consists of foams or felts, a separate cover is not used. Insulation covers are used to hold the insulation batting in place and to keep out contaminants such as dust and fluids, especially water. Very thin plastic films (0.013 to 0.05 mm) of polyimide, polyvinyl fluoride, or poly(aryletherketones) reinforced with nylon or polyester yarns are used extensively due to their low weight and good tear resistance. In areas that are subject to abuse, lightweight, abrasion-resistant coated fabrics such as vinylcoated nylon and vinyl-coated fiber-glass are used. Areas subject to higher temperature require the use of silicone-coated fiber-glass, metal-coated fiberglass, or ceramic covers. Insulation is installed using a variety of attachments, including poly(phenylenesulphide) (PPS) hook and loop fasteners, nylon fasteners, snaps, and splicing tapes. Interior panels Although a few monolithic laminate panels have been used, most panels in airplane interiors of newly manufactured airplanes are sandwich structures having high stiffness-to-weight ratio. These panels are typically constructed of fiber-glass or carbon fabric-reinforced thermosetting resin face sheets adhesively bonded to an aramid (e.g., Nomex or Kevlar, Du Pont) paper honeycomb core, and covered with laminated, decorative thermoplastic surface film or paint that must be colourfast and easy to clean. These panels are used for ceilings, galleys, lavatories, sidewalls, baggage racks, floors, partitions, and closets. All panels for these applications must meet FAR 25.853(a) and (a-1) as they are used in the aircraft. Textiles Decorative textiles are also used to cover interior panels on surfaces that face the passengers on galleys, lavatories, closets, and partitions. Plush, hand-tufted, predominantly wool tapestries are often used on upper panel surfaces. Lightweight carpeting or a grospoint construction is common on lower panel surfaces. A variety of materials and methods are used to make tapestries. The lower surface textile, wainscoting, is usually fabricated of treated wool with a very lightweight backing or no backing. With the heat release regulation, most of these tapestries and wainscoting could no longer be applied. Tapestry and wainscoting fabrics made from synthetics and wool/synthetic combinations are produced in order to meet the heat release requirements. Draperies are used to close off sections of the aircraft such as galleys and to separate the classes of

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passenger service. Drapery fabrics are usually wool or polyester fabric that has been treated with a flame retardant. Floor coverings are vinyl or carpets depending on the location in the aircraft. All floor coverings must meet FAR 25.853(b). Carpet covers most of the cabin floor, including the aisle and under the seats. Most aircraft have wool or nylon-face yarns with polyester, polypropylene, cotton, or fiber-glass backing yarns and a fire-retardant backcoating. Wool-faced yarns are treated with a fire retardant. Nylon carpets must have a highly fire-retardant, back-coating for fire resistance. Carpet under-layers of felt are used in some aircraft for noise suppression. Areas where fluid spills are likely, such as galleys and lavatories, use plastic floor coverings typically made of vinyl with a reinforcing fabric backing and a non-slip surface. Nonmetallic air-ducting Owing to the relative compactness of an aircraft, much of the conditioned air ducting has to be routed around many different parts. This results in some very complex shapes. Nonmetallic ducting is used to create these complex parts. There are three basic types of nonmetallic duct constructions: fiber-reinforced thermosetting resin, thermoplastic, and rigid thermosetting resin foam. All conditioned air ducting must meet FAR 25.853(b). Fiber-reinforced resins used in ducting consist of woven fiber-glass with polyester, epoxy, cyanate ester, or phenolic thermosetting resin systems. Some aromatic polyamide/epoxy is also used. Ducts made from these materials are usually coated after curing on the outside with a polyester or epoxy resin to seal against leaks. Fiber-glass impregnated with silicone rubber is the industry standard for duct boots because of flexibility, strength, low air permeability, and good fire resistance. Thermoplastic ducting is typically made of vacuum-formed polycarbonate or poly(etherimide). Thermoplastic ducts are not as strong or durable as fiberreinforced resin; however, thermoplastic ducts are much less costly to fabricate. Polyimide or poly(isocyanurate) foam ducts are used for larger ducts with complex shapes and have the advantage of not requiring additional insulation. Foam ducts are popular for their low weight. Linings Sheet products are used to cover areas where strength and flexibility are required in a contoured shape; in addition, linings provide an aesthetically pleasing surface and protect the assemblies behind the liner. Areas such as the exit doors, flight deck, cabin sidewalls, door-frames, and cargo holds utilize liners fabricated of reinforced thermosetting resins or thermoplastics. Decorative sidewall liners made of formed aluminium are used in some aircraft. Depending on the application, the liners must meet FAR 25.853(a), (a-1), or (b) or FAR 25.855(a) or (a-1), or a combination. Linings that are subjected to passenger and food cart

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traffic are typically manufactured from plies of fabric-reinforced resin. Their flexibility, impact resistance, high strength, and low weight make them ideal for lower sidewall kick panels. Cargo liners that are required to meet FAR 25.855(a) and (a-1) are typically fabricated from fiber-glass reinforced resins because of the burn-through and impact resistance. Linings that see less abuse and do not require high strength are fabricated from thermoplastics because less expensive fabrication methods can be used. Flight deck sidewalls, upper door liners, attendant stations, and closeouts are typical applications for vacuum, heat and pressure (thermoformed) plastics such as ABS, polycarbonate, poly(etherimide), poly(phenylsulphone), and polyvinyl chloride/acrylic blends. In many applications, thermoplastics are pigmented and textured and do not require any decorative covering. Electrical components Wire and cable insulation consisting of thermosetting resins and thermoplastics comprises a small amount of the nonmetallic material in an aircraft (see Fig. 20.2). For general wire and cable applications inside the pressure shell, the majority of high performance wire jacketing in use today includes radiation cross-linked poly(ethylene-tetrafluoroethylene) (ETFE) and a composite construction of poly(tetrafluoroethylene) (PTFE) wrapped around the conductor, a middle layer wrapped with heat resistant polyimide film, and an outer sheath of PTFE to provide abrasion resistance. In some areas, aromatic polyamide braiding is used to cover power feeder cables for abrasion resistance. For higher temperature and fuel areas, PTFE is used almost exclusively. Where very high temperature or burn-through resistance is a requirement, filled PTFE is typically used. Asbestos had been used as the filler in the past, but has been replaced by proprietary fibers. To withstand the high temperature requirements of fire zones, heavily nickel-plated copper wire is used to ensure continued operation of electrical equipment. All wire insulations in the airplane must meet FAR 25.1713(c) (previously FAR 25.869(a) and FAR 25.1359(d)), and those located in fire zones must also meet FAR 25.1713(b). Conduit and tubing of different types are used for electrical wires and components. Poly(vinyl fluoride) and polyolefin thermoplastic heat shrink tubing, silicone rubber impregnated glass fiber-braid, and extruded and convoluted nylon tubing are industry standards. Most connectors in an aircraft are made of Bakelite with silicone or hardened dielectric material inserts that have no specific FAR burn requirements. Connectors located in firewalls, however, must be fireproof and are made of low-carbon or stainless steel to meet burn-through requirements. Windows At present all aircraft windows are fabricated from biaxially-oriented, bulk polymerized poly(methylmethacrylate) (or stretched acrylic) thermoplastic.

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Stretched acrylic has the optical clarity, strength, low weight, and solvent resistance necessary to meet the service specifications for pressurized aircraft windows in the cabin. All windows must meet flammability requirements of FAR 25.853(b-2). Transparent dust covers that are vented to the cabin interior are used to protect the windows and are made from tough polycarbonate or acrylic thermoplastic that has been hard-coated for scratch resistance. Transparent dust covers must meet FAR 25.853 (a-1). Small parts Except for electrical wire, cable insulation and small parts such as knobs, handles, rollers, fasteners, clips, grommets, rub strips, and pulleys that would not contribute significantly to the propagation of a fire, all parts/materials not identified in FAR 25.853(a), (b), (b-1), or (b-2) must not have a burn rate greater than 10 cm per minute when tested horizontally in accordance with FAR 25.853 (b-3). Figure 20.3 shows the flammability (as average heat release capacity) of commodity, engineering, and heat resistant polymers compared to the flammability of formulated solid plastic materials used in automotive interiors,11 consumer electronics,12 railway car interiors,13 aircraft overhead spaces (hidden areas), and aircraft cabin interior furnishings as measured for commercial samples in a standardized test.14 Heat release capacities of railway car materials were estimated from ref. 13 using empirical correlations.15 It is clear from Fig. 20.3 that the materials used in aircraft cabin interiors are, on average, less

20.3 Average flammability of polymers and plastic products used in various applications. Number of materials tested is in parentheses. Error bars are one standard deviation.

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flammable than those used in railway cars, and that aircraft cabin materials are significantly less flammable than the plastics used for consumer electronics and automotive interiors. The flammability of the materials in Fig. 20.3 reflects the severity of the requirements for each application, e.g. a horizontal Bunsen burner test for automotive,16 a 10-second vertical Bunsen burner for electronics,17 smoke18 and flame spread19 under radiant heat for rail cars.20 The test methods for aircraft overhead and interior materials3,5 are discussed in the following section.

20.4

Fire test methods for aircraft materials3,5

20.4.1 Vertical Bunsen burner (BB) test for cabin and cargo compartment materials This test method, shown in Fig. 20.4, is intended for use in determining the resistance of materials to flame when tested according to the 12-second and 60-

20.4 Vertical Bunsen burner test (photo courtesy of the govmark organization).

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second vertical flame tests specified in Federal Aviation Regulation (FAR) 25.853 and FAR 25.855. In the test, a 10-mm diameter Bunsen or Tirrill burner having a 38-mm methane or similar combustible gas flame is applied to the bottom edge of a 75-mm wide by 305-mm high specimen clamped to expose 51mm of width in a draft free cabinet. The sample is positioned in the flame such that the bottom of the sample is 19 mm above the top of the burner at the start of the test and is exposed to the flame for 12 or 60 seconds, depending on the requirement. At least three specimens are tested for each determination. Individual and average values are reported for the ignition time (nominally 12 or 60 seconds), the flame time of the specimen after removal of the burner, the flame time of drips (if any), and the burn length are reported. The requirements are that the average flame time of all of the specimens tested not exceed 5 seconds for either the 12- or 60-second exposure vertical test, and that the average drip time not exceed 3 seconds for the 60-second test, or 5 seconds for the 12-second test. The average burn length for all of the specimens tested shall not exceed 152 mm (6 in.) for the 60-second exposure or 203 mm (8 in.) for the 12-second exposure. Materials affected (60-second exposure): Interior ceiling panels, wall panels, partitions, galley structure, large cabinet walls, structural flooring and stowage compartments and racks. Materials affected (12-second exposure): Floor covering, textiles including drapery and upholstery, seat cushions, padding, coated fabrics leather, trays and galley furnishings, electrical conduit, thermal and acoustical insulation and insulation covering, air ducting, joint and edge covering, liners of Class B and E cargo or baggage compartments, floor panels of Class B, C or E cargo or baggage compartments, insulation blankets, cargo covers and transparencies, molded and thermoformed parts, air ducting joints, trim strips.

20.4.2 45-Degree Bunsen burner test (45 BB) for cargo compartment liners and waste stowage compartment materials This test is shown in Fig. 20.5 and is intended for use in determining the resistance of materials to flame penetration and to flame and glow propagation when tested according to the 30-second, 45-degree Bunsen burner test specified in FAR Part 25. In the test a 10 mm diameter Bunsen or Tirrill burner having a 38 mm methane or similar combustible gas flame is applied for 30 seconds to the center of the bottom surface of a square specimen clamped to expose a 203 mm wide by 203 mm surface oriented at 45 degrees to the horizontal in a draft free cabinet. At least three specimens are tested for each determination. Individual and average values of the flame time of the specimen after removal of the burner, the glow time, and whether or not flame penetration occurs are reported. The requirements are that the average flame time for all specimens tested not

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20.5 45 degree Bunsen burner test (photo courtesy of the govmark organization).

exceed 15 seconds, that the Bunsen burner not penetrate any of the specimens tested, and that the average glow time not exceed 10 seconds. Materials affected: liners of Class B and E cargo or baggage compartments, floor panels of Class B, C or E cargo or baggage compartments not occupied by crew or passengers.

20.4.3 Horizontal Bunsen burner test for cabin, cargo compartment, and miscellaneous materials This test method (see Fig. 20.6) is intended for use in determining the resistance of materials to flame when tested according to the 15-second horizontal Bunsen burner tests specified in FAR 25.853. In the test a 10-mm diameter Bunsen or Tirrill burner having a 38-mm methane or similar combustible gas flame is applied for 15 seconds to the bottom edge of rectangular specimen clamped to expose a 51 mm wide by 305 mm long horizontal surface in a draft free cabinet. At least three specimens are tested for each determination. The individual and average linear burn rates (mm/min) in the horizontal direction are reported. The requirement is that the average burn rate for the all of the specimens tested not exceed 6.3 cm/minute (FAR 25.853(b-2) or 10 cm/minute (CFR 14 part 25.853(b-3).

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20.6 Horizontal Bunsen burner test (photo courtesy of the govmark organization).

Materials affected: clear plastic windows and signs, parts constructed in whole or in part of elastomeric materials, edge lighted instrument assemblies, seat belts, shoulder harnesses, and cargo and baggage tie-downs including bins, pallets, etc., used in passenger or crew compartments.

20.4.4 60-Degree Bunsen burner test for electrical wire (60 BB) This test method (see Fig. 20.7) is intended for use in determining the resistance of electrical wire insulation to flame when tested according to the 30-second, 60degree Bunsen burner test specified in FAR 25.869. In the test a 10-mm diameter Bunsen or Tirrill burner having a 38-mm methane or similar combustible gas flame is applied for 30 seconds to the center section of wire specimen oriented at 60-degrees with respect to the horizontal. One end is clamped to the horizontal surface while the other end passes over an elevated pulley and is attached to a free weight, the mass of which depends on wire gage. At least three specimens are tested for each determination. The individual and average specimen flame time after removal of the burner, flame time of drips (if any), and burn length are reported. The requirement is that the average flame time not exceed 30 seconds, the average flaming drip time not exceed 3 seconds and that the average burn length not exceed 76 mm.

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20.7 60 (and 45) degree Bunsen burner test (photo courtesy of the govmark organization).

Materials affected: insulation on electrical wire or cable installed in any area of the fuselage.

20.4.5 Heat release rate test for cabin materials This test method is intended for use in determining heat release rates to show compliance with the requirements of FAR 25.853. Heat release is a measure of the amount of heat energy evolved by a material when burned. It is expressed in terms of energy per unit area (J/m2 or kW min/m2). Heat release rate is a measure of the rate at which heat energy is evolved by a material when burned. It is expressed in terms of power per unit area (kW/m2). The maximum heat release rate occurs when the material is burning most vigorously. It has been shown that the maximum heat release rate and total heat released by burning materials in an aircraft cabin are predictors of the time available for escape in the event of a post-crash fire. The apparatus, sometimes referred to as the Ohio State University calorimeter, used to measure rate of heat release has a test

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20.8 Rate of heat release apparatus (photo courtesy of fire testing technology).

chamber that is approximately 20 cm deep by 40 cm wide by 80 cm high fabricated of 0.6 mm thick stainless steel and covered with 2.5 cm of mineral board insulation (see Fig. 20.8). Ambient temperature compressed air is metered into the test chamber at 0.04 m3/s. Thermopiles located at the air inlet and exit ports of the test chamber measure the enthalpy rise of the air stream caused by burning of the sample and are used to compute the heat release rate after calibration with a metered methane flame. A 15 cm square test specimen having the thickness used in the aircraft is tested in a vertical orientation under radiant heat exposure of 35 kW/m2 provided by silicon carbide heating rods. Methane pilot flames are used to force ignition of the sample and burn the gases evolved prior to ignition. Heat release rate is measured for the duration of the test and encompasses the period of ignition and progressive flame involvement of the surface. At least three specimens of each material are tested. The requirement is that the average maximum heat release rate during the five-minute test not exceed 65 kW/m2 and that the average heat released during the first two minutes of the test not exceed 65 kW-min/m2 (3.9 MJ/m2).

Materials with reduced flammability in aerospace and aviation

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Materials affected: interior ceiling and wall panels other than lighting lenses and windows, partitions other than transparent panels needed to enhance cabin safety, galley structure, stowage carts, large cabinets and stowage compartments.

20.4.6 Smoke test for cabin materials This test method is used to determine the smoke generating characteristics of airplane passenger cabin interior materials to demonstrate compliance with the requirements of FAR 25.853. The quantity measured is the specific optical density, which is a dimensionless measure of the amount of smoke produced per unit area by a material when it is burned. The test chamber shown in Fig. 20.9 is a square cornered box with internal dimensions 91 cm wide by 61 cm deep by 91 cm high, containing a sample holder, radiant heat source, and a photometric system capable of detecting light transmittance values down to 1% with an

20.9 Smoke test for cabin materials (photo courtesy of fire testing technology).

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accuracy of 0.03%. In the test, a 65-mm square sample of the thickness used in the aircraft cabin is exposed to a pilot flame and 25 kW/m2 of radiant energy to force ignition and burning. At least three specimens are tested of each material. The requirement is that the average maximum specific optical density measured during the four-minute test not exceed 200, which corresponds to 3% light transmission through the test chamber. Materials affected: interior ceiling and wall panels other than lighting lenses and windows, partitions other than transparent panels needed to enhance cabin safety, galley structure, stowage carts, large cabinets and stowage compartments.

20.4.7 Oil burner test for seat cushions This test method evaluates the burn resistance and weight loss characteristics of aircraft seat cushions exposed to a high intensity open flame to show compliance with the requirements of FAR 25.853. In the test shown in Fig. 20.10, a seat back (vertical) and a seat bottom (horizontal) cushion that are representative of the production articles used in aircraft seats with regard to materials and construction are attached with wires to a steel seat frame and exposed for two minutes to a kerosene or fuel oil burner calibrated to produce 120 kW/m2 at the seat surface. The weight loss of the cushion set during the test is determined by weighing the cushions before and after the test and the burn lengths are measured along the top and bottom surfaces of the vertical and horizontal cushions. At least three cushion sets are tested for each construction. The requirement is that the average weight loss for the cushion sets not exceed 10%

20.10 Oil burner test for seat cushions.

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and that neither the individual nor the average burn lengths of the cushions exceed 43 cm. Materials affected: passenger cabin seat cushions and some lightweight seat constructions.

20.4.8 Oil burner test for cargo liners This test method evaluates the flame penetration resistance capabilities of aircraft cargo compartment lining materials utilizing a high intensity open flame to show compliance with the requirements of FAR 25.855. In the test (see Fig. 20.11), a kerosene or fuel oil burner calibrated to a heat flux of 91 kW/m2 at the sample surface impinges on a specimen of cargo sidewall or ceiling liner that is 61 cm by 41 cm and mounted in a steel frame in the orientation (vertical or horizontal) that it will be used in the aircraft. At least three specimens are tested of each material. The requirement is that none of the specimens allow the flame to penetrate through the thickness or exhibit a temperature at a distance of 10 cm from the rear surface in excess of 204 ëC (400 ëF). Materials affected: ceiling and sidewall liners of Class C cargo or baggage compartments.

20.11 Oil burner test for cargo liners.

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20.4.9 Radiant panel test for flame spread of thermal/acoustic insulation This test method shown in Fig. 20.12 evaluates the flammability and flame propagation characteristics of thermal/acoustic insulation when exposed to both a radiant heat source and a flame to show compliance with FAR 25.856(a). In the test a horizontal specimen 31.8 cm wide by 58.4 cm long is placed inside a test chamber that is 1.4 m long by 0.5 m wide by 0.76 m high in a horizontal position beneath a 32.7 cm by 47 cm gas-fired or electrically heated panel inclined at an angle of approximately 20 degrees with respect to the horizontal and calibrated to provide a radiant heat flux gradient. The heat flux decreases from a maximum value of 17 kW/m2 at the sample leading edge to 16 kW/m2 at a distance of 10 cm along the sample length. The edge of the specimen is ignited for 15 seconds using a propane burner, after which the burner is removed and the flame spread and time of flaming are measured for a minimum of three replicate tests. The requirement is that the flame cannot spread more than 5.1 cm along the sample length (i.e., the critical heat flux for flame spread is about 17 kW/m2) and the flame time after removal of the burner not exceed 3 seconds for any specimen. Materials affected: thermal/acoustic insulation in the fuselage of transport category aircraft.

20.12 Radiant panel test for flame spread of thermal/acoustic insulation (photo courtesy of fire testing technology).

Materials with reduced flammability in aerospace and aviation

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20.4.10 Oil burner test for burn through resistance of thermal/ acoustic insulation This test method evaluates the burn through resistance of thermal/acoustic insulation exposed to a high intensity open flame to show compliance with FAR 25.856(b) Appendix F, Part VII. In the test, shown in Fig. 20.13, two 81 cm by 91 cm samples of thermal/acoustic insulation are mounted side-by-side in a steel frame and exposed to a kerosene or fuel oil burner calibrated to a heat flux of 180 kW/m2 at the sample surface. The time to flame penetration (if any) and the heat flux behind the sample are measured. At least three specimen sets are tested of each material. The requirement is that none of the specimens allow the flame to penetrate through the thickness for four minutes or register a heat flux of greater than 23 kW/m2 at a distance of 31 cm behind the sample face. Materials affected: thermal/acoustic insulation in the lower half of the fuselage of transport category aircraft. Figure 20.14 is a cross-sectional diagram of the aircraft cabin showing the cabin interior, cargo area and unoccupied areas and the fire test regulations governing the materials in these areas.

20.13 Oil burner test for burn-through resistance of thermal/acoustic insulation.

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20.14 Fire tests by aircraft cabin location.

20.5

Future trends

The following initiatives are under way at the US Federal Aviation Administration to address or upgrade materials fire safety on aircraft.

20.5.1 Upgraded requirements for materials in unoccupied areas This program was established to address components installed inside the pressurized hull of the aircraft in areas not visible or readily accessible to passengers and crew. The fire test procedures and acceptance criteria for these `hidden materials' are being reviewed to determine whether more stringent requirements are needed.

20.5.2 Fireproof aircraft cabin In 1995, a long-range fire research program was initiated to develop a fireproof aircraft cabin.21 Ultra fire resistant materials that resist ignition and burning during flight and in a post-crash, fuel-fed fire were identified as a means to achieve this goal. Research is fundamental in nature and seeks to understand the burning process of combustible solids at the molecular level. Applications for ultra fire resistant materials in commercial aviation include thermoplastics for seat parts and transparencies, thermosetting resins for composite wall panels and air ducts, textile fibers for carpets and seat fabrics, and rubber for foam seat cushions, hoses, and sealants.

Materials with reduced flammability in aerospace and aviation

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20.5.3 Special condition for polymer composite fuselage Industry is currently seeking certification for new design transport category aircraft having a fuselage constructed largely of carbon fiber-reinforced epoxy composites. These composite structures are difficult to ignite and tend to burn slowly22 because the flammable component is less than 24% of the composite weight after charring of the epoxy is taken into account. There are no fireworthiness requirements for airplane fuselages because they have traditionally been constructed of aluminium, which is noncombustible under normal conditions. Consequently, the composite fuselage aircraft will need to be certified under a Special Condition after demonstrating an equivalent level of safety.

20.6

Acknowledgements

The author is indebted to Jim Peterson of Boeing, and Jeff Gardlin, Gus Sarkos, Pat Cahill and John Reinhardt of the FAA for their help with the manuscript, and to Richard Walters for all of the heat release capacity measurements. Certain commercial equipment, instruments, materials and companies are identified in this paper in order to adequately specify the experimental procedure. This in no way implies endorsement or recommendation by the Federal Aviation Administration.

20.7

References

1. Final Report to President Clinton of the White House Commission on Aviation Safety and Security, February 12, 1997. 2. C.P. Sarkos, An Overview of Twenty Years of R&D to Improve Aircraft Fire Safety, Fire Protection Engineering, 5, 4±16 2000. 3. United States Code of Federal Regulations, Title 14: Aeronautics and Space, Volume 1, Chapter I: Federal Aviation Administration, Parts 1±49, 2007. 4. J.M. Peterson and S.E. Campbell, Regulatory and Industry Requirements for Fire Safe Aircraft Interior Materials: Materials and Process Affordability ± Keys to the Future, Proceedings of the 43rd International SAMPE Symposium and Exhibition, Anaheim, CA, May 31±June 4, pp. 1594±1599, 1998. 5. Aircraft Material Fire Test Handbook, Federal Aviation Administration, Report DOT/FAA/AR-00/12, April 2000. 6. W.T. Westfield, The Role of Research and Development on Safety Regulation, Federal Aviation Administration Final Report DOT/FAA/AR-95/84, October 1995. 7. J.M. Peterson, Flammability Tests for Aircraft, in Flammability Testing of Materials Used in Construction, Transport and Mining, V.B. Apte, Ed., Woodhead Publishing Ltd., Cambridge, England, 2006, pp. 275±301. 8. J. Troitzsch, International Plastics Flammability Handbook, 2nd Edition, Hanser Publishers, Munich, Germany, 1990. 9. Fire Safety Aspects of Polymeric Material, Volume 6, Aircraft: Civil and Military, National Materials Advisory Board, Publication NMAB 318-6, Washington, D.C., 1977, p. 68.

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10. Fire- and Smoke-Resistant Interior Materials for Commercial Transport Aircraft, National Materials Advisory Board Report, NMAB-477-1, National Academy Press, Washington, DC 1995. 11. R.E. Lyon and R.N. Walters, Flammability of Automotive Plastics, Society of Automotive Engineers (SAE) World Congress, Detroit, MI, April 3±6, 2006. 12. A.B. Morgan, Micro Combustion Calorimeter Measurements on Flame Retardant Polymeric Materials, Proceedings of the Society for the Advancement of Materials and Process Engineering (SAMPE) Technical Conference, Cincinnati, OH, October 29±November 1, 2007. 13. R.D. Peacock and R.W. Bukowski, Flammability Tests for Railway Passenger Cars, in Flammability Testing of Materials Used in Construction, Transport and Mining, V.B. Apte, Ed., Woodhead Publishing Ltd., Cambridge, England, 2006, pp. 336± 360. 14. Standard Test Method for Determining Flammability Characteristics of Plastics and Other Solid Materials Using Microscale Combustion Calorimetry, ASTM D 730907, American Society for Testing and Materials (International), West Conshohocken, PA, 2007. 15. R.E. Lyon, R.N. Walters and S.I. Stoliarov, Screening Flame Retardants for Plastics Using Microscale Combustion Calorimetry, Polymer Engineering & Science, 47, 1501±1510 (2007). 16. U.S. Code of Federal Regulations, Title 49: Transportation, Volume 6, Chapter V, National Highway Traffic Safety Administration, Part 571 Federal Motor Vehicle Safety Standards, Standard 302, Flammability of Interior Materials (FMVSS 302), October 1, 2003. 17. Flammability of Plastic Materials, UL 94 Section 3 (Vertical: V-0/1/2), Underwriters Laboratories, Northbrook, IL, 1991. 18. Standard Test Method for Specific Optical Density of Smoke Generated by Solid Materials, ASTM E 662, American Society for Testing and Materials (International), West Conshohocken, PA, 2006. 19. Standard Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source, ASTM E 162, American Society for Testing and Materials (International), West Conshohocken, PA, 2006. 20. U.S. Code of Federal Regulations, Title 49: Transportation, Volume 4, Chapter II, Federal Railroad Administration, Part 238 Passenger Equipment Safety Standards, Standard 103, Fire Safety (FRA 238.103), October 1, 2006. 21. R.E. Lyon, Advanced Fire Safe Aircraft Materials Research Program, Final Report DOT/FAA/AR-95/98, January 1996. 22. J.G. Quintiere, R.N. Walters and S. Crowley, Flammability Properties of Aircraft Carbon Fibre Structural Composites, Final Report DOT/FAA/AR-07/57, Federal Aviation Administration, October 2007.