Aviation Safety Recommendations
Date Issued: 28 August 2001
The Honourable David Michael Collenette, P.C., M.P.
Minister of Transport
Ms. Carol Carmody
National Transportation Safety Board
Jean Overney, Chief Inspector
Swiss Aircraft Accident Investigation Bureau
Subject: Material Flammability Standards
The Circumstances of the Swissair Flight 111 Accident
On 02 September 1998, Swissair Flight 111 (SR 111), a McDonnell Douglas MD-11 aircraft, was travelling from New York to Geneva with 215 passengers and 14 crew on board. Approximately 53 minutes after take-off, as the aircraft was cruising at flight level 330, the crew noticed an unusual smell in the cockpit. Within about three and a half minutes, the flight crew noted some smoke in the cockpit and declared the international urgency signal "Pan Pan" to Moncton Air Traffic Services. SR 111 was cleared to the Halifax airport from a position 57 nautical miles to the southwest. While the flight crew was manoeuvring the aircraft in preparation for the landing in Halifax, they were unaware that there was a fire spreading above the ceiling in the front area of the aircraft. About 11 minutes after the initial assessment by the crew that some visible smoke was present, the situation in the cockpit began to deteriorate rapidly. The autopilot disconnected and the aircraft's flight data recorder began to record a rapid succession of anomalies that reflected failures related to various aircraft systems. The flight crew declared an "emergency", and indicated a need to land immediately. Within about a minute thereafter, or about 12 minutes after the initial assessment of the existence of some visible smoke in the cockpit, radio communications and secondary radar contact with SR 111 was lost while the aircraft was in level flight at about 10 000 feet above sea level. About six minutes later, the aircraft crashed into the ocean near Peggy's Cove, Nova Scotia, Canada, fatally injuring all 229 occupants.
Since the aircraft crashed into water, all fire damage occurred in flight. The ongoing investigation (A98H0003) has identified substantial fire damage above the drop-down ceiling in the forward section of the aircraft extending about 1.5 metres forward and 5 metres aft of the cockpit wall. Although the origin of the fire has not been determined, the investigation has revealed several safety deficiencies with respect to standards for material flammability. The elimination of these deficiencies would reduce the probability of loss of life resulting from in-flight fires.
In August of 1999 the Transportation Safety Board of Canada (TSB) issued two aviation safety recommendations.(1) These recommendations addressed safety deficiencies associated with the propensity of thermal acoustic insulation blankets covered with metallized polyethylene terephthalate (MPET) to propagate fire.(2) The recommendations focussed on the test criteria stipulated in the United States Federal Aviation Regulations (FARs) for the certification of such materials. Subsequently, the United States Federal Aviation Administration (FAA) issued airworthiness directives(3) mandating the removal of MPET-covered blankets from aircraft registered in the United States. Additionally, the FAA proposed regulatory changes that would require more rigorous testing of all thermal acoustic insulation materials.(4)
Despite these initiatives, the TSB is concerned that there remain safety deficiencies in the material flammability standards, and that these pose an unacceptable risk to the flying public. First, in a series of aviation safety recommendations issued in December 2000 and entitled In-Flight Firefighting, the Board stated that material flammability standards for aeronautical products are an integral component of any in-flight firefighting "system". The Board is concerned that the flammability standards for certain materials used in the pressurized portion of an aircraft are inadequate.(5) Second, despite many initiatives to mitigate electrical wire discrepancies (including action taken subsequent to the issuance of TSB Aviation Safety Advisory 980031-1, 22 December 1998), the Board believes that the certification test criteria for aircraft wires do not adequately address the potential for wire failures to ignite or propagate fires. Third, indications that the failure of certain aircraft systems, such as crew oxygen, could exacerbate a fire in progress suggest that current requirements for conducting system safety failure analysis may be inadequate.
In summary, the Board's aviation safety recommendations address these safety deficiencies:
- the inadequacy of flammability standards for certain aircraft materials;
- the inadequacy of aircraft wire certification testing; and
- the inadequacy of system safety analysis to address the effects of potential system failures created by on-board fires.
Material Flammability Standards
The investigation has assessed the flammability characteristics of the materials present in areas of the SR 111 aircraft damaged by fire, and the regulations and guidelines that apply to the certification of those materials. The most significant material flammability deficiency discovered has been the inappropriate flammability characteristics of the MPET-covered thermal acoustic insulation blankets. Other certified materials, discussed in Appendix A, also exhibit undesirable fire-propagation characteristics. The analysis of how these materials__either alone or in concert__may have contributed to the initiation and progress of the SR 111 fire is complex, and is ongoing. However, the flammability characteristics of the materials involved, and the speed with which the fire damage occurred, raise questions about the existing standard of flammability required for materials used in the fabrication of aeronautical products.
For the most part, civil aviation authorities (CAAs) maintain their own material flammability standards, and there are slight regulatory variations among national CAA jurisdictions. However, the standards are based on, or similar to, those described in the FARs, and this discussion will be confined to the material flammability standards specified by the FARs. These regulatory standards are the minimum required for certification of aircraft. Although not required by regulations, manufacturers routinely impose supplemental testing on materials used in their products.
In general, each aircraft material must be tested to demonstrate its tendencies both to ignite and to propagate flame. The FAA has developed a series of tests which, in principle, are designed to represent the fire environment to which a given material may be exposed.(6) The FAA expectation is that one or more fire tests must be conducted on each material as a prerequisite to certification.(7) The number and severity of flammability tests required for a particular material largely depend on three criteria: the intended location of the material within the aircraft, the type, and the quantity. For example, materials used in one location, such as in partitions in occupied cabin interiors, may be subjected to more rigorous testing than materials used in other locations, such as some unoccupied spaces. Also, parts constructed of a particular type of material, such as elastomeric materials, may be subjected to less stringent tests, regardless of their intended location. Finally, the more of a particular material installed in an aircraft (either in greater quantities or in larger components), the more stringent the testing required for the material's certification.
Related Research and Development
Regulations are based upon ongoing efforts in research and development (R&D), which seek to continually improve fire safety in aviation. This R&D is primarily based on three factors: analysis of accidents and incidents, emerging technology, and new aircraft designs.(8) Although these efforts are international in scope, historically, the FAA has functioned as the lead agency as a direct consequence of its mandate.(9) Material flammability standards form an integral part of this R&D effort.
Current regulations are the result of efforts made over many years to utilize finite R&D resources to maximize safety improvements. In 1975-76 the FAA commissioned a study to determine the feasibility of, and the tradeoffs between, two basic approaches to providing fire safety improvements to the modern, wide-bodied transport fuselage.(10) Two approaches were investigated as part of that study:
- the application of the latest available technologies in early warning fire-detection and fire-extinguishing systems (described as a "fire management system"); and
- the application, in the cabin interior, of improved materials offering high fire-retardant qualities and low emissions of smoke and toxic gas.
The study concluded that there were merits and limitations in each approach, and that an approach combining a fire management system with selective material improvements may offer the most potential for providing timely fire protection in all cases.(11)
Ultimately, the thrust of R&D did not fully pursue this combined approach, and only limited follow-up research was conducted into the concept of developing an on-board fire management system.(12) It was reasoned that in-flight fires are rare, and typically originate in hidden and inaccessible areas; therefore, a limited use of the fire management concept would suffice. The best protection against in-flight fires, it was concluded, would be achieved through the targeted use of materials that have high fire-containment and ignition-resistance properties. It was concluded that such materials, combined with the selective use of early and reliable detection and efficient suppression techniques, would provide the required level of protection. R&D related to in-flight fires has led to increased fire protection in areas such as cargo compartments and lavatories.
While certain initiatives were taken to address the threat from in-flight fires, such as those mentioned above, the FAA's main R&D focus in the 1980s was towards increasing survivability in a post-crash fire environment. This R&D effort was, and continues to be, based on a post-crash scenario involving an intact fuselage adjacent to a fire that is sustained by uncontained aviation fuel. Full-scale burn tests using this scenario concluded that a post-crash fire within the aircraft would be sustained primarily by burning cabin interior materials. This FAA research also concluded that incapacitation of any potential survivors was primarily dependant upon toxic gases generated by a phenomenon known as "flashover".(13) At flashover, conditions rapidly deteriorate to a level at which survival is unlikely.(14) The inference__not universally accepted__is that the threat to occupants from combustion smoke and toxic/irritant gases, before flashover occurs, does not warrant the introduction of material toxicity standards. As a consequence, subsequent R&D has concentrated on developing improved flammability standards for cabin interiors, to delay the onset of flashover and thereby increase survivability. These efforts have resulted in major improvements to flammability standards for selected cabin materials, such as seat cushion fire-blocking layers and panels that release low levels of heat and/or smoke.
Consequence of Current Regulations Concerning Flammability Standards
Based on the above, under current FAA regulations, the most stringent material flammability standards are reserved for large surface panels (such as sidewalls, ceilings, stowage bins and partitions) in the occupied areas of the aircraft. Flammability standards for materials used in the remainder of the aircraft interior are less stringent.
The FARs specify the level of fire protection required, based primarily on the location of a material within an aircraft. For most of the materials used outside the occupied areas of the cabin, the performance criteria are defined by the "horizontal Bunsen burner test" for miscellaneous materials, as specified in Appendix F to FAR Part 25 Part I.(15) Unlike other fire-testing methods, which measure flame time and burn length to establish a material's capacity to self-extinguish, the horizontal burn test only measures a material's rate of burn. For material that is subjected solely to the horizontal burn test, its only known flammability characteristic is whether it will burn at or below a pre-determined rate.(16) If such materials are not required to be self-extinguishing, they must be flammable(17)and capable of sustaining or propagating fire. Furthermore, as the highest flammability standards are reserved for large surface panels in occupied areas of the cabin, it is likely that the most flammable materials will be in remote, hidden, and inaccessible areas of an aircraft. Yet these are the areas where a variety of electrical ignition sources may initiate an in-flight fire, and where there are the fewest defences in terms of detection and suppression.
Summary of Current FAR Requirements
The current FAR requirements, as described above, result in the following material flammability hierarchy:
- flammable materials with an acceptable rate of burn;
- materials that will self-extinguish within an acceptable flame time and burn length; and
- selected cabin materials that will self-extinguish and release no more than a predetermined amount of heat and smoke.
Therefore, many aircraft materials currently in use are either flammable, or will burn within established performance criteria.
Additional Fire-Related Testing
Only a limited number of materials, most of which are used in the passenger cabin, are certified using additional tests for smoke generation and heat release. Yet some in-flight fires have shown that smoke will migrate to the occupied areas of the aircraft and can impede the crew's ability to effectively deal with the associated emergency (see examples in Appendix B). Furthermore, within present regulations, no material is required to pass a certification fire test that measures toxicity. Beyond meeting a standard of flame time and burn length, there is no regulatory requirement to determine additional flammability characteristics for many materials used in aircraft.
Information on how materials not tested for flammability characteristics, such as heat release, smoke generation and toxicity, may contribute to the severity of an in-flight fire is contained in Appendix C. However, as these flammability characteristics are by-products of the combustion process, the Board believes that the most effective means to mitigate these additional threats is to eliminate the use of all materials that sustain or propagate fire.
Existing material flammability standards allow the use of flammable materials as well as materials that propagate flame within predetermined limits. In addition to the associated fire risk, the majority of these materials pose additional hazards, as there is no regulation requiring that other flammability characteristics__such as heat release, smoke generation and toxicity__be measured. Currently, the most stringent fire tests are reserved for materials located in accessible cabin areas. As a consequence, some of the most flammable materials within the pressurized portions of an aircraft are located in hidden, remote or inaccessible areas. These areas pose a high risk of being involved in potentially uncontrollable in-flight fires.
The Board believes that the use of a material__regardless of its location, type, or quantity__that sustains or propagates fire when subjected to realistic ignition scenarios,(18) constitutes an unacceptable risk, and that, as a minimum, material used in the manufacture of any aeronautical product should not propagate or sustain a fire in any realistic operating environment. Therefore, the Board recommends that:
For the pressurized portion of an aircraft, flammability standards for material used in the manufacture of any aeronautical product be revised, based on realistic ignition scenarios, to prevent the use of any material that sustains or propagates fire.
Rating: Fully Satisfactory
Material Flammability Test Requirements for Aircraft Wiring
Large modern aircraft may contain more than 250 kilometres of wire of various sizes and insulating materials. Some digital flight control systems rely totally on wire interconnections, rather than the cables and pulleys used in earlier designs. The quantity and importance of electrical wire in aircraft is increasing.
During a detailed examination of the SR 111 wreckage, 20 electrical copper wires were found that displayed melted copper caused by an arcing event.(19) The significance of the arcs, in terms of whether or not they initiated the SR 111 fire, is under review; the possibility has not been ruled out. A review of data produced by the FAA, the Airline Pilots Association and Boeing shows that electrical systems have been a factor in approximately 50% of all aircraft occurrences involving smoke or fire, and that wiring has been implicated in about 10% of those occurrences. Significant examples of such occurrences can be found in Appendix D.
Unlike most materials used in the construction of aeronautical products, which are passive until involved in a fire, the failure of aircraft wiring has the capacity to play an active role in fire initiation. The failure of insulation material on a powered wire may create a high temperature arcing event and thereby ignite adjacent materials. However, despite the potential for wire to initiate a fire, the only material flammability test mandated for the certification of aircraft wire, including its associated insulation material, is the "60º Bunsen burner test."(20) This test method is designed to measure the burn length and extinguishing time of a given wire's insulation material. In effect, the sole material flammability performance criterion mandated for aircraft wire insulation material is the determination of how a single unpowered wire will behave when involved in a fire in progress. This is essentially the same basic flammability characteristic that is known about most passive materials used in the pressurized portion of the aircraft.
Typically, an aircraft wire that initiates an arcing event has sustained some preliminary damage. Damage such as cracks, cuts, stretching, contamination, and chafing can result in a breakdown of the insulation material, thereby exposing the conductor. While such damage is considered serious and would demand a repair, in many cases it can go undetected. An exposed conductor can exist indefinitely with little or no adverse effect on aircraft performance. It is only when the exposed conductor is "shorted" that an arcing event occurs.
Notwithstanding the special attention that is paid to the design, installation, and maintenance of aircraft wiring systems, wiring irregularities can develop in any aircraft. On 22 December 1998 the TSB issued Aviation Safety Advisory 980031-1, which detailed various MD-11 wiring anomalies discovered during many aircraft inspections. These anomalies included the following: chafed, cracked, broken, or cut electrical and bonding wires; inconsistencies in the routing of wires and wire bundles; loose terminal connections; excessively small wire bend radii; and unsealed electrical wire conduits. Subsequently, the FAA issued many wire-related airworthiness directives (for various aircraft, including the MD-11) as part of its MD-11 Wiring Corrective Action Plan.
Additionally, the FAA commissioned a Transport Aircraft Intrusive Inspection Project as part of its Aging Transport Systems Rulemaking Advisory Committee. This project inspected six recently retired transport category aircraft from a variety of manufacturers and operators. The study discovered wires degraded through poor repairs or splices, heat-damaged or burnt wire, vibration damage or chafing, cracked insulation, arcing, and insulation delamination.(21) The report concludes that there are risks associated with uncorrected degenerative conditions, and recommends options for prevention or mitigation of such failures. While increasing the frequency and quality of maintenance inspections is a viable option, since most of the wiring system is bundled and located in hidden or inaccessible areas, it is difficult to monitor the health of an aircraft's wiring system during scheduled maintenance with existing equipment and procedures. Therefore, it is realistic to expect that until wire maintenance inspection equipment and methods are perfected, wire failures that could result in fire ignition will continue to occur.
The electrical protection of an aircraft's wiring system is provided by a variety of circuit protective devices. The most common of these are circuit breakers, which are designed to protect the electrical distribution system__the wires__from an electrical overload. However, circuit breakers have design limitations. An overload caused by a wire failure may not lead the circuit breaker to de-energize the circuit; this may create high heat and a potential ignition. While ongoing R&D seeks to improve circuit protection devices, at this time there are portions of aircraft wiring systems that may not be protected against all electrical overload conditions.
Irrespective of efforts to design, install and maintain an aircraft's wiring system to a high standard, deficiencies with wires will likely persist and present the potential for wire failures. While all wires will arc under certain circumstances, the dynamics of how a particular wire fails during an arcing event is highly dependant on the composition of the wire insulation.(22) Understanding the dynamics of how a wire will fail under realistic conditions would be valuable, given the known consequences of the failure of an energized wire. While the FAA endorses several failure tests (for example, the dry arc tracking test procedure), it does not require any failure tests as a basis for wire certification.
The Board believes that, given the incidence of aircraft wire failures and their role as potential ignition sources, the absence of a certification requirement that measures a wire's failure characteristics, and that specifies performance standards under realistic operating conditions, constitutes a risk. Therefore, the Board recommends that:
A certification test regime be mandated that evaluates aircraft electrical wire failure characteristics under realistic operating conditions and against specified performance criteria, with the goal of mitigating the risk of ignition.
Rating: Satisfactory in Part
System Evaluation: Fire Hardening Considerations
Various materials, including endcaps from both the oxygen and air conditioning systems used in the MD-11, have exhibited less-than-ideal fire propagation characteristics as described in Appendix A. The premature failure of either the aluminium endcap used in the crew oxygen system, or the elastomeric endcaps used on ducts within the air conditioning system, would likely have exacerbated the in-flight fire on board SR 111. Under current regulations, a material's intended location and application must be identified in order to define which fire tests are required for that material's certification. If a material is to be used in a designated fire zone (e.g., the engine compartment) it must be hardened to withstand the more rigorous conditions associated with that environment and so delay a failure that might contribute to a fire in progress.(23) In most other areas of the aircraft, there is no requirement to determine if a material's failure would exacerbate a fire in progress. Yet the selection of inappropriate materials may lead to premature breaches of certain systems__such as oxygen, hydraulic, wiring, and air environmental__which could exacerbate an in-flight fire.
It is an established aviation industry practice to consider the consequences of a system's failure during the certification process. FAR 25.1309 requires that a system safety analysis be conducted as part of a system's certification process. The purpose of such an analysis is to confirm that the system has been designed and installed using a fail-safe methodology.(24) This approach ensures that equipment failures will not have any adverse effect on an aircraft's safe flight and landing. Typically, this analysis does not include an assessment of the consequences of the system's failure as a result of fire. For example, the certification of oxygen systems whose design includes materials with dissimilar properties, without consideration for how this arrangement would affect the integrity of the system when it is exposed to a fire, may allow a latent failure to persist. Similarly, where an air conditioning duct system is made of dissimilar materials (such as aluminium ducts with elastomeric endcaps), an in-flight fire may cause an elastomeric endcap to fail before the aluminium portion of the same duct system. This failure of the endcap material would introduce forced air into a fire in progress and would have the potential to aggravate the fire. Assessing the impact of a system's failure when exposed to fire, and designing aircraft systems to delay failures that could seriously aggravate an in-flight fire, would provide an additional defence in limiting the size and progress of in-flight fires.
The Board believes that a fire-induced material failure in some aircraft systems has the potential to augment the combustion process and exacerbate the consequences of an in-flight fire. Therefore, the Board recommends that:
As a prerequisite to certification, all aircraft systems in the pressurized portion of an aircraft, including their sub-systems, components, and connections, be evaluated to ensure that those systems whose failure could exacerbate a fire in progress are designed to mitigate the risk of fire-induced failures.
As the investigation proceeds, should the Board identify additional safety deficiencies in need of urgent attention, it will make further aviation safety recommendations.
On behalf of the Board
Flammability testing done as part of the A98H0003 investigation has revealed that some certified materials used in the MD-11 exhibit less-than-ideal fire propagation characteristics:
- Hook-and-Loop Fastener System: This material is a lightweight fastener system that is employed in a variety of applications throughout the aircraft. It is used in both occupied and remote areas of the aircraft. No documentation has been discovered to indicate exactly what certification testing was used to approve this material. Subsequent testing by the TSB and the FAA discovered that this material demonstrated unacceptable flame propagation characteristics when tested alone or when used as part of a typical thin-film thermal acoustic insulation material installation.
Elastomeric Material: Such materials are made from various polymers and have the elastic properties of natural rubber. Amongst
other applications, elastomeric materials are used on the MD-11 to cap unused duct openings in the air conditioning system. While no certification
documents have been discovered, Appendix F to Part 25 Part I of the FARs requires that such elastomeric material be tested in accordance with the
horizontal Bunsen burner test. TSB/FAA testing has revealed that although the material passes the horizontal test, when tested using the vertical
Bunsen burner test it was a qualified "failure".
Initially, when samples of elastomeric endcap material were tested using the vertical Bunsen burner test they glowed rather than flamed. The material would be considered compliant as the test criteria allow for the material to glow and still pass the test. However, when the test was allowed to continue, the glowing material eventually burst into flame, which entirely consumed the material.
- Aluminium Endcaps: The original oxygen line installation on the MD-11 was constructed entirely of aluminium tubing. The system included a "capped" line designed for use in a different configuration of the MD-11. Due to installation difficulties encountered during aircraft manufacture, the original aircraft manufacturer replaced the aluminium with a steel line during aircraft production. For undetermined reasons, the aluminium endcap was not replaced by a steel one at this time. Concerned about how this heterogeneous configuration would behave during a high-temperature event, the TSB conducted several experiments. During the testing, the system leaked, and in some instances the endcap failed completely, allowing a free flow of oxygen. Such an oxygen leak during an in-flight fire might be catastrophic.
- Insulation Blanket Tape: Some thin-film thermal acoustic insulation blanket constructions require the use of adhesive tape. Typically, the tape is made of a material similar to that of the blanket cover. During manufacture, although the blanket cover material was required to pass the vertical Bunsen burner test, no flammability testing was required for the Douglas Material Specification 1984, Type 4 tape used on the accident aircraft. Subsequent fire testing conducted by the TSB and the FAA has discovered that this type of tape exhibits unacceptable fire propagation characteristics.
- Polyethylene Foam: Such materials are used for a variety of applications throughout the aircraft, in many different shapes and sizes. In preliminary testing on some examples of this type of material, it has demonstrated a tendency to propagate flame. No certification documentation has been discovered that would indicate how these materials were certified. To fully characterize the material's flammability properties, further testing is anticipated.
- Composite Ducts: Parts of the accident aircraft's air conditioning system located in the area of heavy fire damage were constructed of composite material. The investigation is interested in determining which flammability tests were conducted on this material during the certification process. The TSB and FAA are planning fire tests to determine this material's flammability properties.
Synopses of aircraft fires in which cockpit visibility was a factor:
- 18 January 1990: The cockpit of an MD-80 was filled with smoke from overheated electrical wire insulation. The left generator phase B power feeder cable terminal had melted from intense arcing. Additionally, smoke was generated when the molten metal sprayed and ignited adjacent material. (United States National Transportation Safety Board)
- 16 October 1993: In an MD-81, after levelling at flight level 180, smoke from an electrical source entered the cockpit from behind the overhead panel. Shortly thereafter, the smoke became so dense that the pilots were unable to read emergency checklists or the instrument approach procedures. Investigators determined that the smoke was caused by a massive smoldering fire involving the emergency power switch. (Aircraft Accident Investigation Bureau, Germany)
- 08 August 2000: A DC-9 experienced an in-flight fire in which the captain and first officer noticed a smell of smoke shortly after takeoff. The crew immediately donned oxygen masks and smoke goggles. The smoke became very dense and restricted the crew's ability to see either the cockpit instruments or the visual references outside the aircraft. Investigation found extensive heat damage to wires and insulation in the electrical panel behind the captain's seat. The heat was sufficient to blister the primer on the fuselage crown skin. (United States National Transportation Safety Board)
- 01 October 2000: An MD-80 experienced an electrical fire approximately 15 minutes into the flight, the cockpit filled with smoke and a loud popping sound was heard accompanied by sparks from the jump seat area. The examination of the aircraft disclosed a 2 by 1½ inch fire-damaged hole in the left jump seat wall. Several heavy-gauge electrical wires were welded together on the opposite side of the wall. There were also four 50-ampere circuit breakers popped on the left circuit breaker panel behind the pilot's seat. (United States National Transportation Safety Board)
Heat release is a measure of the amount of heat emitted by a burning material. How quickly a fire reaches flashover depends on the rate of heat release of the combustibles involved. Certain materials used in the occupied areas of the cabin must demonstrate that they will not exceed a specified maximum heat release rate and maximum total heat release. The purpose of this requirement is to delay the onset of flashover during a post-crash fire, as there is a direct correlation between a material's heat release and its contribution to the onset of flashover. In contrast, much of the flammable material that is likely to be involved in an in-flight fire is located in remote areas, such as "attic" spaces. Airflow considerations aside, compartments within aircraft can promote the accumulation of hot gases and combustion by-products, thereby creating conditions conducive to flashover.
Under existing regulations, materials other than selected cabin materials are not required to pass any heat-release test. The inference is that delaying flashover in the event of a fire in these unoccupied locations is not viewed by regulatory authorities as a safety improvement requiring additional regulation. Requiring all materials to meet a heat-release standard would provide an increased resistance to flashover and benefits comparable to those currently applicable to selected cabin materials. The Board has concerns about the lack of broader standards to limit the amount of heat that would potentially be released by burning materials within aircraft; it believes that the associated risks could be mitigated by eliminating the use of materials that sustain or propagate fire.
Material smoke-generation requirements are designed to measure the amount of smoke emitted by burning materials. The primary objective in limiting smoke generation is to maintain visibility for egress during a post-crash fire. Therefore, smoke tests are typically only required for selected materials used in occupied areas of the cabin. There is no smoke test requirement for the majority of materials in the rest of the aircraft. In-flight fires, examples of which are contained in Appendix B, indicate that smoke will migrate to the occupied areas of the aircraft and can impede the crew's ability to effectively deal with such an emergency. The effect, on the passengers, from prolonged exposure to smoke generated during an otherwise survivable in-flight fire event is largely unknown. As there are presently no provisions designed to isolate passengers from such smoke, reduced visibility during ensuing ground evacuations can be anticipated. Establishing a certification standard limiting smoke generation for all aircraft materials would increase visibility and survivability.
The Board has concerns about the lack of standards regarding smoke generation associated with burning aircraft materials; it believes that the smoke-related risks could be mitigated through the elimination of materials that sustain or propagate fire.
Materials designated for use in aircraft are not required, by regulation, to meet any toxicity standards, although manufacturers can impose toxicity criteria of their own. Regulatory requirements and strategies have focussed on improving the chances of passenger survival in the event of a post-crash fire. This is accomplished by mandating that selected cabin materials meet heat-release standards that delay the onset of flashover. This approach reflects the belief that a material's toxic effects will not be a factor until after flashover. As the flashover phenomenon is generally considered a non-survivable event, the argument is made that there is limited benefit in establishing a toxicity standard for burning materials. The physiological effects of inhaling the toxic by-products likely to be present in a post-crash fire prior to flashover, on a passenger's ability to evacuate the aircraft, are considered minimal. However, passenger evacuation is not an option in an in-flight fire. While the flight crew may be able to take limited measures to evacuate some smoke from the cabin, aircraft occupants must cope with the potentially debilitating effects of toxic and irritant gases emitted by burning aircraft materials.
As discussed in the TSB's aviation safety recommendations A00-16 to A00-20, a crew has only a limited ability to effectively assess and suppress such hidden, inaccessible fires. Therefore, in its incipient stages, the most likely in-flight fire scenario would involve an uncontrolled fire comprising known flammable materials. As there are no mandated toxicity criteria for materials used within aircraft, some of these materials are likely toxic when burned. Such toxic by-products would be spread by the air circulation within the pressurized hull and could eventually impair crew and passengers. While it can be argued that the crew are equipped with breathing apparatus that allows them to continue to function, passengers have no such equipment. The passenger oxygen delivery system is designed to be used in a depressurization event and will not protect the user against smoke or airborne toxins. In fact, the MD-11 Aircraft Operations Manual warns that passenger oxygen masks must not be released below 14 000 feet when smoke or an abnormal heat source is present, as the oxygen may increase the possibility or severity of a cabin fire.(25)
Some in-flight fires have been resolved with minimal on-board firefighting coupled with immediate action to land the aircraft (with flight crew smoke masks donned). However, immediate access to an emergency airport may not always be an option, such as during a transoceanic flight. In such cases, passengers could suffer from prolonged exposure to combustion by-products with an unknown effect on their ability to survive. The Board has concerns about the lack of standards to limit the amount of toxic emissions that would potentially be released by burning materials within an aircraft. It believes that the associated risks could be mitigated by eliminating the use of materials that sustain or propagate fire.
Synopses of several occurrences in which aircraft wiring was a factor:
- 24 November 1993: An MD-87 was taxiing when smoke was detected in the cabin. A fire subsequently erupted and destroyed the aft cabin interior. Investigators concluded that two chafed wires suffered metal-to-metal contact with the frame, igniting surrounding material. (United States National Transportation Safety Board)
- 17 July 1996: A Boeing 747 experienced an in-flight breakup. Investigators found that the breakup was caused by an overpressure event in the centre wing tank. This overpressure was the result of a Jet A fuel/air vapour explosion. The investigation concluded that the most likely ignition event was a short circuit outside of the centre wing fuel tank that entered the tank through electrical wire associated with the fuel quantity indication system. (United States National Transportation Safety Board)
- 28 November 1998: A Boeing 747 returned to its departure airport after an apparent fault associated with an electrical and electronic equipment (E&E) compartment cooling system ground exhaust valve. Investigators discovered several arced wires in a small wire bundle associated with the exhaust valve. Insulation blanket cover material had subsequently ignited and was consumed by fire. (Air Accidents Investigation Branch, United Kingdom)
- 22 December 1998: A Lockheed L-1011 experienced electrical wire arcing inside an avionics compartment where a wire bundle had sustained wire-to-wire arcing. The wire bundles were also saturated with fluid. (United States National Transportation Safety Board)
- 29 March 1999: Maintenance personnel discovered evidence of a fire on board an MD-11 while inspecting the aft floorboards during a maintenance check. Inspection revealed that a wire bundle had arced to the aircraft frame and ignited the surrounding thermal acoustic insulation material. The insulation cover material had entirely burned away. (United States National Transportation Safety Board)
29 December 2000: A Lockheed L-1011 experienced an electrical fire forward of the flight engineer's station in which an arc was observed at the location of the windshield heat wire bundle above the first officer's side window. Examination of the affected wires revealed electrical arcing had occurred between the aircraft structure, a clamp, and a 30-wire bundle, in which 20 wires were burned. (United States National Transportation Safety Board)
- 10 January 2001: A Boeing 767 landed in foggy conditions at Salt Lake City, Utah. At or shortly after touchdown, several circuit breakers popped, an electrical wire bundle in the E&E bay shorted out, and a small fire broke out causing smoke in the cockpit. The aircraft taxied to the gate uneventfully, and the smoke stopped when the engines were shut down. (United States National Transportation Safety Board)
- A99-07 and A99-08 dated 11 August 1999
- Polyethylene terephthalate film is often referred to as Mylar, a registered trademark of E.I. du Pont de Nemours and Company. Other manufacturers have metallized the film for use as a thermal acoustic insulation blanket cover material, which is known as metallized polyethylene terephthalate.
- FAA dockets 99-NM-161-AD and 99-NM-162-AD
- Notice of Proposed Rulemaking Docket No. FAA-2000-7909
- For the purposes of this discussion, the pressurized portion of the aircraft, or pressure vessel, includes cockpit, cabin, avionic compartments, cargo compartments, and the various accessory spaces between the passenger compartment and the pressure hull.
- Aircraft Materials Fire Test Handbook, DOT/FAA/AR-00/12, April 2000
- The FAA's Airworthiness Standards contain performance requirements for the certification of aircraft. For Transport Category Aircraft, FAR Part 25 applies. Because the review of aircraft components for compliance to the FAR flammability requirements is only done in conjunction with the certification of an entire aircraft, the regulator uses these standards to approve the whole aircraft together with its integrated component parts as opposed to approving the individual aircraft parts in isolation.
- Constantine P. Sarkos, "Future Trends in Aircraft Fire Safety Research and Development," presentation at the International Aircraft Fire Cabin Safety Conference, Atlantic City, N.J., 16-20 November 1998.
- The FAA is mandated to conduct fundamental research related to aircraft fire safety in accordance with the Aviation Safety Research Act of 1988.
- Feasibility and Tradeoffs of a Transport Fuselage Fire Management System, Report No. FAA-RD-76-54, June 1976.
- An integrated fire management system is one that incorporates fire detection, monitoring, and suppression throughout the aircraft.
- Aircraft Command in Emergency Situations (ACES) Phase 1: Concept Development, DOT/FAA/CT-90/21, April 1991.
- Constantine P. Sarkos, "An Overview of Twenty Years of R&D to Improve Aircraft Fire Safety," Fire Protection Engineering, Number 5, Winter 2000.
- For the purposes of this document, flashover is defined as a sudden and rapid spread of fire within an enclosure.
- The horizontal Bunsen burner test is one in which a horizontally mounted specimen is exposed to a Bunsen burner flame for 15 seconds. The average burn rate is recorded.
- Depending on the material's application, the performance criteria as described in the horizontal Bunsen burner test require the rate to be at a maximum of either 2.5 or 4.0 inches/minute.
- For the purpose of this discussion, a flammable material is defined as one that is susceptible to combustion to the point of sustaining or propagating a flame.
- The use of a realistic ignition scenario requires an assessment of the possible ignition sources, including a fire in progress and other factors that could affect the fire environment to which the material may be subjected.
- The melting point of copper is 1083ºC.
- FAR 25.869 requires that a single unpowered wire be mounted at 60º to a flame for a specified time in accordance with Appendix F of Part 25.
- Transport Aircraft Intrusive Inspection Project Final Report prepared by the Intrusive Inspection Working Group, 29 December 2000.
- Patricia L. Cahill and James H. Dailey, Aircraft Electrical Wet-Wire Arc Tracking, FAA Final Report, DOT/FAA/CT-88/4, 1988.
- For the purposes of this discussion, hardening means taking due consideration, during the design stage, to accommodate unfavourable environmental conditions, such as heat.
- The use of a fail-safe methodology for system evaluation ensures that the system is designed so that it is capable of compensating automatically and safely for a failure.
- MD-11 Aircraft Operations Manual, Emergency Equipment: Oxygen, 5.0 Limitations effective 07 February 1991
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