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Safety Recommendation Details

Safety Recommendation A-96-174
Details
Synopsis: ON 7/17/96, ABOUT 2031 EASTERN DAYLIGHT TIME, A BOEING 747-131, N93-119, OPERATED AS TRANS WORLD AIRLINES FLIGHT 800 (TWA800), CRASHED INTO THE ATLANTIC OCEAN, ABOUT 8 MILES SOUTH OF EAST MORICHES, NEW YORK, AFTER TAKING OFF FROM JOHN F. KENNEDY INTERNATIONAL AIRPORT (JFK), JAMACIA, NEW YORK. ALL 230 PEOPLE ABOARD THE AIRPLANE WERE KILLED. THE AIRPLANE, WHICH WAS OPERATED UNDER TITLE 14 CODE OF FEDERAL REGULATIONS (CFR) PART 121, WAS BOUND FOR CHARLES DE GAULLE INTERNATIONAL AIRPORT (CDG), PARIS, FRANCE. THE FLIGHT DATA RECORDER (FDR) & COCKPIT VOICE RECORDER (CFR) ENDED SIMULTANEOUSLY, ABOUT 13 MINUTES AFTER TAKEOFF. EVIDENCE INDICATES THAT AS THE AIRPLANE WAS CLIMBING NEAR 13,800 FEET MEAN SEA LEVEL (MSL), AN IN-FLIGHT EXPLOSION OCCURRED IN THE CENTER WING FUEL TANK (CTW); THE CWT WAS NEARLY EMPTY.
Recommendation: TO THE FEDERAL AVIATION ADMINISTRATION: Require the development of and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel-air mixtures in the fuel tank: (a) significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems & the addition of insulation between heat-generating equipment & fuel tanks. Appropriate modifications should apply to newly certificated airplanes &, where feasible to existing airplanes.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Action
Mode: Aviation
Location: EAST MORICHES, NY, United States
Is Reiterated: No
Is Hazmat: No
Is NPRM: No
Accident #: DCA96MA070
Accident Reports: In-flight Breakup Over the Atlantic Ocean Trans World Airlines Flight 800, Boeing 747-141, N93119
Report #: AAR-00-03
Accident Date: 7/17/1996
Issue Date: 12/13/1996
Date Closed: 10/16/2008
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s):

Safety Recommendation History
From: NTSB
To: FAA
Date: 10/16/2008
Response: NMC# 103225: In its investigation report on the TWA flight 800 accident, the Safety Board concluded that a fuel tank design and certification philosophy that relied solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, was fundamentally flawed because experience had demonstrated that all possible ignition sources cannot be predicted and reliably eliminated. Despite industry’s reservations about the affordability and practicality of developing and installing a flammability reduction system, the FAA pursued a development program that demonstrated such a system to be affordable, practical, and effective for in-service and newly manufactured aircraft. Based on its success in this undertaking, on July 21, 2008, the agency moved to implement the requirement. The Safety Board is aware that this needed safety improvement was controversial and was opposed by a number of industry groups and that enactment of the rule was due to the personal involvement and commitment of staff at the FAA from the working level to the highest levels within the Department of Transportation. The final rule published by the FAA requires that the fuel/air mixtures in all fuel tanks be below a prescribed flammability level for all newly manufactured aircraft that have more than 30 seats, as well as modifications to passenger-carrying aircraft manufactured after January 1, 1992, to achieve the same level of protection. The regulation defines the limits for the potentially explosive mixture and explains how the level is determined; however, the regulation does not mandate the use of a particular technology, such as nitrogen inerting, to achieve a reduction in fuel tank flammability to comply with the rule. The FAA action establishes a level of acceptable risk from a flammable fuel/air mixture in the tanks. The Safety Board notes that a level of risk remains, especially in the wing/unheated tanks; however, the FAA has examined the risk and evaluated it to be within the limits for acceptable risk in 14 Code of Federal Regulations Section 25.1309. The Safety Board’s review of the final rule’s implementation schedule revealed that approximately 2,700 in-service airplanes will need to be modified and that the rule allows manufacturers 2 years to design the needed systems and begin production of the equipment needed for this new technology. After the manufacturers have completed their design work, operators will have 7 years to make the mandated changes; however, the FAA requires that 50 percent of an operator’s fleet of affected aircraft be modified and using the system within 4 years after the manufacturers have completed their design changes. In addition, if operators modify their operations specifications (operational procedures that are requirements) to include operational changes that will immediately reduce the flammability of fuel tanks, such as the use of ground-conditioned air, the operator will receive a 1-year extension to the mandated compliance time for the aircraft modifications. Issuance of the final rule on July 21, 2008, meets the intent of Safety Recommendation A-96-174, which is classified CLOSED -- ACCEPTABLE ACTION. The Safety Board thanks the FAA and its staff for their persistent efforts to demonstrate that practical and effective design changes were possible to preclude the operation of airplanes with explosive fuel/air mixtures in fuel tanks, and for developing and implementing a regulation to require these changes.

From: FAA
To: NTSB
Date: 10/23/2007
Response: Letter Mail Controlled 10/24/2007 8:00:32 AM MC# 2070605: - From Robert A. Sturgell, Acting Administrator: I &I writing to provide you with the latest information about what the Federal Aviation Administration and the Department of Transportation have done and continue to do to reduce the chance of an in-flight fuel-tank explosion. I can assure you that we have taken the lessons of the TWA 800 tragedy very seriously, and the FAA is actively working on a comprehensive approach to address this important safety problem as it pertains to aircraft with heated center wing tanks. As you know, we have already done a great deal using a two-pronged approach; elimination of ignition sources and fuel tank inerting. Immediately following TWA-800 we began issuing Airworthiness Directives (ADS) to eliminate unsafe conditions, as they were identified, that could lead to an ignition in the fuel tank. In 2001, we issued Special Federal Aviation Regulation No. 88 to require manufacturers to analyze their fuel tank designs to identify any potential system failures that could cause ignition and design corrective actions. That effort led to issuance of additional ADS. Despite all our work to eliminate ignition sources, we continue to find unanticipated failures and maintenance errors that generate ignition sources. We now know that analysis will never capture all the ways that a spark could be generated. That led us to recognize the need for the second prong of our approach. We are actively working to complete a rulemaking action that would set requirements for reducing fuel tank flammability. In 2005 we proposed a standard that reduces the level of flammability in the fuel tank. We believe that the marketplace and the creativity of engineers will result in options for meeting that standard. The most practical method we currently know will provide adequate protection is the addition of inert gas (nitrogen) to displace the flammable vapors in the fuel tank. I am sure you are aware that fuel tank inerting is costly. We continue to work on the details of the rulemaking action to determine how it can be imposed consistent with our mandate to demonstrate that the additional benefits of this requirement are sufficient to justify its costs. There may be ways to limit the costs, and we are obliged to consider them. We believe that a cost-justifiable rule is possible, however, and the FAA is diligently working to fine tune the regulatory analysis.

From: NTSB
To: FAA
Date: 3/23/2006
Response: Notation 6788N: The National Transportation Safety Board has reviewed the Federal Aviation Administration’s (FAA) Notice of Proposed Rulemaking (NPRM), titled “Reduction of Fuel Tank Flammability in Transport Category Airplanes; Proposed Rule,” which was published in 70 Federal Register 70921-70962 on November 23, 2005. The notice proposes new rules that will require operators and manufacturers of transport-category airplanes to take steps that, in combination with other required actions, should greatly reduce the chances of a catastrophic fuel-tank explosion. The new rules, if adopted, will not specifically require operators and manufacturers to adopt inerting technology but, rather, will establish performance-based requirements that set acceptable levels of flammability exposure in tanks most prone to explosion or require the installation of an ignition mitigation means in an affected fuel tank. Following the July 17, 1996, fuel tank explosion that destroyed Trans World Airlines flight 800, the National Transportation Safety Board issued Safety Recommendation A 96-174, which is on the Safety Board’s List of Most Wanted Transportation Safety Improvements and asked, in part, that the FAA do the following: Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks: (a) Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. [emphasis added] After Safety Recommendation A-96-174 was issued, the FAA convened two aviation rulemaking advisory committee (ARAC) working groups to obtain industry input. The first ARAC working group produced a final report in 1998 recommending that the FAA continue with efforts to mitigate ignition sources, adding that the FAA should also continue to research nitrogen-inerting options. The FAA required an analysis of fuel tank systems in most types of transport airplanes, issued numerous airworthiness directives to address potential ignition sources found in existing airplanes, and required similar changes in the design of new airplanes. However, the potential for additional ignition sources continued to exist, and the second working group, in 2000, was tasked with evaluating inerting methods. Although the second ARAC did not arrive at a method to protect fuel tanks, the FAA continued research that led to development of a viable protection system. The subject NPRM is the culmination of the FAA’s efforts to address this safety issue. Although the Safety Board supports the NPRM, the Board believes that more needs to be done to specifically preclude the operation of transport-category airplanes with explosive fuel/air mixtures in fuel tanks, as stated in Safety Recommendation A-96-174. The Board is concerned, for example, that the performance-based approach proposed in the NPRM allows a minimum average flammability exposure. The Board is aware of the first ARAC’s finding that fuel tanks located in the wings cool more quickly than some airplanes’ center wing fuel tanks, which can be heated by proximity to warmer areas in the airplane, and that, consequently, fuel tanks located in the wings have a significantly lower average flammability exposure or ignition risk. However, the fact remains that wing fuel tanks have exploded, and Safety Recommendation A-96-174 was not limited to certain types of fuel tanks, or to tanks with specific types of exposure, or to tanks with explosive risks that vary or lessen over time. Regarding the application of means to reduce fuel tank flammability, such as nitrogen inerting, the Safety Board agrees with the FAA that fuel tanks that retain a higher internal temperature due to ambient conditions are at higher risk and should be addressed first. However, the Board also believes that the FAA should at least provide design guidance to encourage the use of available nitrogen generation technologies for all fuel tanks, including those that are normally cooled by colder ambient regimes. On aircraft equipped with this technology, once ullage space in the heated tanks is filled with nitrogen-enriched air, nitrogen production will be available for the remaining fuel tanks. Venting it overboard would be a waste of an available resource, especially in light of the ease with which the FAA has found distribution can be accomplished. The Board is aware that computer modeling has indicated that some nitrogen may reach the other fuel tanks if shared portions of the vent system exist. The Safety Board also notes that, although not intended for missile defense or entirely effective as such, flammability reduction systems could mitigate the results of shrapnel entering fuel tanks following a terrorist act. The industry has only recently become concerned about the threat of missile attacks on civilian airplanes. Published information about the November 2003 attack on the DHL Airbus A-300 in Baghdad, Iraq, illustrates the flammability exposure of wing fuel tanks as a result of shrapnel. The Safety Board is aware of estimates of potential attacks that were cited to support missile protection schemes for civilian airliners and believes that the cost-benefit analysis for the NPRM should include these estimates. Further, the Safety Board does not believe that cargo airplanes should be omitted from this proposed rulemaking. Cargo airplanes typically carry few crewmembers but regularly use airports in densely populated areas. Although the potential for an accident on any particular flight is not great, the magnitude of a fuel tank explosion during operations near a major airport could be catastrophic. Finally, the Safety Board notes that additional benefits from onboard flammability reduction systems can be realized, but that the FAA does not address any of these. Nitrogen-producing equipment can be used to provide fire protection for avionics, electrical compartments, unoccupied spaces, and cargo compartments. For example, on February 8, 2006, a DC-8 cargo airplane was on fire when it landed at Philadelphia, Pennsylvania, International Airport. The fire was quickly spreading in the cargo area, and the crew may have had only a few extra minutes in which to land the airplane. An inerting system may have suppressed or extinguished the fire, significantly improving the safety of the flight. Further, the introduction of nitrogen-producing technology can eliminate the need for Halon-based fire suppression systems and potable water compressors, reducing maintenance costs and weight. The Safety Board believes that the NPRM’s cost-benefit analysis does not accurately reflect these additional potential safety benefits that will result from the proposed rule. In summary, the Safety Board supports the proposed rulemaking for both existing and new production airplanes but does not believe it goes far enough to fully protect against fuel tank flammability and does not address the additional benefits of nitrogen-producing equipment. We look forward to the fastest implementation of this NPRM and the Board’s suggested revisions, consistent with Safety Recommendation A-96-174. We appreciate the opportunity to comment on this important proposed rule.

From: FAA
To: NTSB
Date: 3/1/2006
Response: In its 3/1/2006 annual report to Congress, Regulatory Status of the National Transportation Safety Board's "Most Wanted" Recommendations to the Department of Transportation, the DOT wrote: The FAA has aggressively pursued research in cooperation with industry that has led to the development of a practical nitrogen inerting-based fuel tank flammability reduction means. This system can significantly reduce the flammability exposure of high-risk fuel tanks. In February 2004, the Administrator announced plans to propose rulemaking that would require flammability reduction means on new and existing airplanes with high-risk fuel tanks. The NPRM was published in the Federal Register on November 23.2005. Boeing designed an inerting-based flammability reduction means for the Boeing 747 high-risk fuel tanks based on the results ofthe FAA research. The FAA approved the Boeing design on the 747 airplane in August 2005. and then Boeing delivered the first production 747 airplane equipped with the flammability reduction means. Boeing delivered the first 8-737 NG equipped with a flammability reduction means on December 8. Southwest Airlines received the first delivery with newly certified flammability reduction means. The next aircraft to have the flammability reduction means is the B-777. The FAA firmly believes that inerting-based flammability reduction means, together with additional ignition prevention measures required as a result of SFAR 88, provide a balanced approach to fuel tank safety that will essentially eliminate the risk of fuel tank explosions.

From: NTSB
To: FAA
Date: 7/28/2005
Response: Notation 6788L: The National Transportation Safety Board has reviewed your notice of proposed special conditions (NPSC), titled “Proposed Special Conditions: Boeing Model 737-200/200C/300/400/500/600/700/700C/800/900 Series Airplanes; Flammability Reduction Means (Fuel Tank Inerting),” which was published in 70 Federal Register 34702 on June 15, 2005. The notice proposes special conditions for Boeing 737 airplane modifications that incorporate a flammability reduction system (FRS) that uses a nitrogen generation system to minimize the fleet’s exposure to a flammable mixture in the center wing fuel tank. The 737 NPSC is the latest of the Federal Aviation Administration’s (FAA) ongoing efforts to address this safety issue and follows a similar NPSC that the FAA developed for the 747 fleet (Docket No. NM270). The Safety Board has investigated center wing fuel tank explosions in two 737 airplanes. The first occurred on May 11, 1990, in Manila, Philippines, and the second occurred on March 3, 2001, in Bangkok, Thailand. With respect to the scope of the problem in the 737 fleet, more than 5,200 of these airplanes are currently in service. Before the July 17, 1996, accident involving TWA flight 800, fuel tank protection was primarily accomplished by identifying and eliminating potential ignition sources. In the final accident report for TWA flight 800, the Safety Board concluded that this design and certification philosophy is fundamentally flawed and that reliance on elimination of potential ignition sources alone is not sufficient. As a result, the Safety Board issued the following safety recommendation to the FAA on December 13, 1996: Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks: (a) Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. (A-96-174) As a result of Safety Recommendation A-96-174, the FAA convened two aviation rulemaking advisory committee (ARAC) working groups to review history, hazards, and potential solutions. The FAA also reviewed the existing fleet of transport-category airplane fuel systems for potential hazards, which has resulted in 68 airworthiness directives (AD) to date. The FAA initiative led to a two-layered approach, requiring both continued elimination of potential ignition sources and development of a nitrogen-based FRS, on which the Boeing-developed system discussed in this NPSC is based. The NPSC requires that overall fuel tank flammability be limited to 3 percent of the fleet average operating time. Conditions in which FRS performance cannot maintain an inert tank ullage include those times the airplane is dispatched with the system inoperative and during expedited descent. Although the Safety Board agrees with the FAA that the combination of flammability reduction and ignition source prevention is the most effective means to prevent fuel tank explosions, the Board has two primary concerns, which are similar to its concerns regarding the 747 NPSC. First, the Safety Board does not agree with the FAA’s premise that wing fuel tanks offer an acceptable minimum level of flammability exposure and is therefore concerned about using this level as a design limit for development of inerting systems. The Board’s position on this issue, previously stated in the TWA flight 800 accident report and in comments submitted in response to Notice of Proposed Rulemaking 99-18, is that flammability exposure in all tanks should be reduced to the lowest level technically feasible. Second, the Safety Board takes exception with the NPSC’s definition of “inert,” which establishes a maximum oxygen concentration of 12 percent at sea level. Past research conducted to support development of military aircraft inerting systems has shown that fuel vapors are combustible at this concentration. These systems, designed to protect against high-energy (intentional) ignition threats, have established 9 percent as an acceptable oxygen concentration to prevent ignition. However, even in civilian transport operations, accidental ignition sources, such as short-circuited generator cables or other system failures, atmospheric lightning strikes, and external threats, may all deliver energy that exceeds the levels that the NPSC indicates are beyond the intended capabilities of the proposed system. Therefore, the Board believes that the 12 percent value should be considered as a “level of reduced flammability” rather than “inert” and that the 9 percent concentration level should be the long-term goal. The Safety Board recognizes the technical challenges and practicality of systems designed for complete elimination of flammable vapors using today’s technology. Although the NPSC has set conditions that fall short of complete elimination of flammable vapors in all tanks at all times, these conditions do represent a dramatic improvement in flammability reduction. The Board notes that including discussions in Appendix 2 that address flight mission distribution (which is a method to determine the mission length for each flight) and fuel tank thermal characteristics are an improvement in the application of the NPSC. We believe that, at the current time, this NPSC is consistent with the Board’s position to reduce flammability to the greatest extent technically feasible while not losing sight of completely eliminating flammability as industry experience and technology improve. Lastly, as with the 747 NPSC, the NPSC for the 737 proposes that the master minimum equipment list (MMEL) permit operation with an inoperative flammability reduction system for up to 10 days/60 flight hours. The Safety Board agrees with the FAA that (1) this is “a safety system that should be operational to the maximum extent practical,” and (2) “…the shortest practical MMEL relief interval should be proposed.” The Board notes that the FAA and industry have adopted a 3-day MMEL relief interval for other inoperative safety systems, such as flight data recorders. Therefore, the Safety Board believes that 10 days is an excessive MMEL relief interval for the flammability reduction system and reiterates that 3 days would be an appropriate interval. Although more than 8 years have passed since Safety Recommendation A-96-174 was issued, the Safety Board again commends the FAA for taking the initiative in exploring nitrogen-inerting concepts and systems for transport airplanes. The Safety Board also commends the FAA for assisting industry implementation of similar systems, such as Boeing has proposed for the 737 and 747 center wing fuel tanks. The Board appreciates the opportunity to comment on these proposed special conditions.

From: FAA
To: NTSB
Date: 2/1/2005
Response: In its 2/1/2005 annual report to Congress, Regulatory Status of the National Transportation Safety Board's "Most Wanted" Recommendations to the Department of Transportation, the DOT wrote: The FAA has aggressively pursued research in cooperation with industry that has led to the development of a practical nitrogen inerting-based fuel tank flammability reduction system. This system can significantly reduce the flammability exposure of high-risk fuel tanks. In February 2004, the Administrator announced plans to propose rulemaking that would require flammability reduction means on new and existing airplanes with high-risk fuel tanks. We expect to issue the notice of proposed rulemaking in early 2005. Boeing has applied for certification of inerting-based flammability reduction means for its airplanes with high-risk fuel tanks and plans to incorporate them on new production aircraft voluntarily beginning in 2005. The FAA firmly believes that inerting-based flammability reduction, together with additional ignition prevention measures required as a result of SFAR 88, can provide a balanced approach to fuel tank safety that will essentially eliminate the risk of fuel tank explosions.

From: NTSB
To: FAA
Date: 8/20/2004
Response: Notation 7655: The National Transportation Safety Board has reviewed the European Aviation Safety Agency’s (EASA) “Proposed Special Conditions on A380 Fuel Tank Safety” and attachments, which address the minimization of ignition sources and fuel tank flammability for the A380 airplane. The Safety Board is concerned that the proposed special conditions (SC) contradict certain recommendations and findings that resulted from the Board’s investigation of the July 17, 1996, accident involving TWA flight 800. On December 13, 1996, the following safety recommendation was issued to the Federal Aviation Administration (FAA): Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks: (a) Significant consideration should be made to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. (A-96-174) The Safety Board notes that the draft SC specifically states that the level of protection called for in Safety Recommendation A-96-174 will not be required, despite the FAA’s request to Airbus Industries that inerting systems be incorporated. The draft SC justifies this decision by citing an aviation rulemaking advisory committee (ARAC) 2001 finding that “inerting was not technically mature.” This statement ignores the fact that since the 2001 ARAC finding, cost-effective prototype nitrogen inerting systems have been developed, implemented, and successfully demonstrated on Airbus and Boeing airplanes. Further, supplemental type certificates have been issued for such systems to be installed as standard equipment on transport airplanes. At the conclusion of its 4-year investigation into the loss of TWA flight 800 and the vulnerabilities of aircraft fuel tanks, the Safety Board released Aircraft Accident Report NTSB/AAR-00/03, which contained the following findings: Failure modes and effects analyses and fault tree analyses should not be relied upon as the sole means of demonstrating that an airplane’s fuel tank system is not likely to experience a catastrophic failure. (finding 19) A fuel tank design and certification philosophy that relies solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated. (finding 20) Operating transport-category airplanes with flammable fuel/air mixtures in fuel tanks presents an avoidable risk of an explosion. (finding 21) The draft SC is inconsistent with these findings. It is based on a philosophy that accepts fuel tank flammability, proposes that safety assessments be performed to demonstrate that the presence of an ignition source within the fuel system is “extremely improbable,” and describes the operation of a new transport airplane with a flammable fuel/air mixture in the fuel tanks. In the 8 years since the TWA flight 800 accident, considerable advances have been made in understanding the problems of, and solutions to, fuel tank flammability and the prevention of fuel tank explosions. Most significantly, we believe that the present state of demonstrated inerting technology is sufficiently mature and that new airplane certification requirements must consider the dramatically lower flammability levels of inerted tanks as a currently feasible and achievable goal for minimizing fuel tank flammability. The Safety Board appreciates the opportunity to comment on this proposed special condition.

From: NTSB
To: FAA
Date: 4/20/2004
Response: Notation 6788I: The National Transportation Safety Board has reviewed your Notice of Proposed Special Conditions (NPSC), titled “Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/ 100B SUD/400/400D/400F Airplanes; Flammability Reduction System (Fuel Tank Inerting),” which was published in 68 Federal Register 68563 on December 9, 2003. The notice proposes special conditions for Boeing 747 airplane modifications that incorporate a flammability reduction system that uses a nitrogen generation system to minimize the fleet’s exposure to a flammable mixture in the center wing fuel tank. As a result of the National Transportation Safety Board’s investigation of the July 17, 1996, accident involving TWA flight 800, the following safety recommendation was issued to the Federal Aviation Administration (FAA) on December 13, 1996: Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks: (a) Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. (A-96-174) The NPSC is the latest of the FAA’s ongoing efforts to address this safety issue. After Safety Recommendation A-96-174 was issued, the FAA convened two aviation rulemaking advisory committee (ARAC) working groups to obtain industry input. The first ARAC working group (the Fuel Tank Harmonization Working Group [FTHWG]), in 1998, produced a final report recommending that the FAA continue with efforts to mitigate ignition sources, adding that the FAA should also continue to research nitrogen-inerting options. The second working group, in 2000, was tasked with evaluating inerting methods. The FAA initiative eventually led to development of a prototype system on which the Boeing-developed system discussed in the NPSC is based. The Safety Board agrees with the FAA’s acknowledgement that ignition source prevention alone has not been completely effective at preventing fuel tank explosions. Further, the Board acknowledges that the current state-of-the-art in inerting system technology does not offer a practical solution for complete elimination of flammable vapors in aircraft fuel tanks. Thus, the Board believes that the combination of flammability reduction and ignition source prevention is the most effective means to prevent fuel tank explosions. With regard to this NPSC, the Safety Board has two primary concerns—the acceptance of a minimum flammability exposure level and the establishment of a maximum oxygen concentration required to inert the fuel tank ullage space. Regarding the first issue, the Safety Board does not agree with the FAA’s premise that wing fuel tanks offer an acceptable minimum level of flammability exposure and is therefore concerned about using this level as a design limit for development of inerting systems. The Board’s position on this issue, previously stated in the TWA flight 800 accident report and in comments submitted in response to Notice of Proposed Rulemaking 99-18, is that flammability exposure in all tanks should be reduced to the lowest level technically feasible. The Safety Board also takes exception with the NPSC’s definition of “inert,” which establishes a maximum oxygen concentration of 12 percent at sea level. Past research conducted to support development of military aircraft inerting systems has shown that fuel vapors are combustible at this concentration. These systems, designed to protect against high-energy (intentional) ignition threats, have established 9 percent as an acceptable oxygen concentration to prevent ignition. However, even in civilian transport operations, accidental ignition sources, such as short-circuited generator cables or other system failures, atmospheric lightning strikes, and external threats, may all deliver energy that exceeds the levels that the NPSC indicates are beyond the intended capabilities of the proposed system. Therefore, the Board believes that the 12 percent value should be considered as a “level of reduced flammability” rather than “inert” and that the 9 percent concentration level should be the long-term goal. We do recognize the technical challenges and practicality of systems designed for complete elimination of flammable vapors using today’s technology. Although the NPSC has set conditions that fall short of complete elimination of flammable vapors in all tanks at all times, these conditions do represent a dramatic improvement in flammability reduction. We believe that, at the current time, this NPSC is consistent with the Board’s position to reduce flammability to the greatest extent technically feasible. The Board recommends that the FAA not lose sight of completely eliminating flammability as industry experience and technology improve. Lastly, according to the preamble of the NPSC, Boeing has proposed that the master minimum equipment list (MMEL) permit operation with an inoperative flammability reduction system for up to 10 days/60 flight hours. The Safety Board agrees with the FAA that (1) this is “a safety system that should be operational to the maximum extent practical,” and (2) “…the shortest practical MMEL relief interval should be proposed.” The Board notes that the FAA and industry have adopted a 3-day MMEL relief interval for other inoperative safety systems, such as flight data recorders. Therefore, the Safety Board believes that 10 days is an excessive MMEL relief interval for the flammability reduction system and that the established limit of 3 days would be an appropriate interval. Although 7 years have passed since Safety Recommendation A-96-174 was issued, the Safety Board again commends the FAA for taking the initiative in exploring nitrogen-inerting concepts and systems for transport airplanes. The Safety Board also commends the FAA for assisting industry implementation of similar systems, such as Boeing has proposed for the 747 center wing fuel tank. The Board appreciates the opportunity to comment on this proposed special condition.

From: NTSB
To: FAA
Date: 2/20/2004
Response: The National Transportation Safety Board has reviewed your Notice of Proposed Special Conditions (NPSC), titled "Special Conditions: Boeing Model 747-100/200B/200F/200C/SR/SP/ 1 OOB SUD/400/400D/400F Airplanes; Flammability Reduction System (Fuel Tank Inerting)," which was published in 68 Federal Register 68563 on December 9, 2003. The notice proposes special conditions for Boeing 747 airplane modifications that incorporate a flammability reduction system that uses a nitrogen generation system to minimize the fleet's exposure to a flammable mixture in the center wing fuel tank. As a result of the National Transportation Safety Board's investigation of the July 17, 1996, accident involving TWA flight 800, the following safety recommendation was issued to the Federal Aviation Administration (FAA) on December 13, 1996: Require the development and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel/air mixtures in the fuel tanks: (a) Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks. Appropriate modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. (A-96-174) The NPSC is the latest of the FAA's ongoing efforts to address this safety issue. After Safety Recommendation A-96-174 was issued, the FAA convened two aviation rulemaking advisory committee (ARAC) working groups to obtain industry input. The first ARAC working group (the Fuel Tank Harmonization Working Group [FTHWG]), in 1998, produced a final report recommending that the FAA continue with efforts to mitigate ignition sources, adding that the FAA should also continue to research nitrogen-inerting options. The second working group, in 2000, was tasked with evaluating inerting methods. The FAA initiative eventually led to development of a prototype system on which the Boeing-developed system discussed in the NPSC is based. The Safety Board agrees with the FAA's acknowledgement that ignition source prevention alone has not been completely effective at preventing fuel tank explosions. Further, the Board acknowledges that the current state-of-the-art in inerting system technology does not offer a practical solution for complete elimination of flammable vapors in aircraft fuel tanks. Thus, the Board believes that the combination of flammability reduction and ignition source prevention is the most effective means to prevent fuel tank explosions. With regard to this NPSC, the Safety Board has two primary concerns-the acceptance of a minimum flammability exposure level and the establishment of a maximum oxygen concentration required to inert the fuel tank ullage space. Regarding the first issue, the Safety Board does not agree with the FAA's premise that wing fuel tanks offer an acceptable minimum level of flammability exposure and is therefore concerned about using this level as a design limit for development of inerting systems. The Board's position on this issue, previously stated in the TWA flight 800 accident report and in comments submitted in response to Notice of Proposed Rulemaking 99-18, is that flammability exposure in all tanks should be reduced to the lowest level technically feasible. The Safety Board also takes exception with the NPSC's definition of "inert," which establishes a maximum oxygen concentration of 12 percent at sea level. Past research conducted to support development of military aircraft inerting systems has shown that fuel vapors are combustible at this concentration. These systems, designed to protect against high-energy (intentional) ignition threats, have established 9 percent as an acceptable oxygen concentration to prevent ignition. However, even in civilian transport operations, accidental ignition sources, such as short-circuited generator cables or other system failures, atmospheric lightning strikes, and external threats, may all deliver energy that exceeds the levels that the NPSC indicates are beyond the intended capabilities of the proposed system. Therefore, the Board believes that the 12 percent value should be considered as a "level of reduced flammability" rather than "inert" and that the 9 percent concentration level should be the long-term goal. We do recognize the technical challenges and practicality of systems designed for complete elimination of flammable vapors using today's technology. Although the NPSC has set conditions that fall short of complete elimination of flammable vapors in all tanks at all times, these conditions do represent a dramatic improvement in flammability reduction. We believe that, at the current time, this NPSC is consistent with the Board's position to reduce flammability to the greatest extent technically feasible. The Board recommends that the FAA not lose sight of completely eliminating flammability as industry experience and technology improve. Lastly, according to the preamble of the NPSC, Boeing has proposed that the master minimum equipment list (MMEL) permit operation with an inoperative flammability reduction system for up to 10 days/60 flight hours. The Safety Board agrees with the FAA that (1) this is "a safety system that should be operational to the maximum extent practical," and (2) "...the shortest practical MMEL relief interval should be proposed." The Board notes that the FAA and industry have adopted a 3-day MMEL relief interval for other inoperative safety systems, such as flight data recorders. Therefore, the Safety Board believes that 10 days is an excessive MMEL relief interval for the flammability reduction system and that the established limit of 3 days would be an appropriate interval. Although 7 years have passed since Safety Recommendation A-96-174 was issued, the Safety Board again commends the FAA for taking the initiative in exploring nitrogen-inerting concepts and systems for transport airplanes. The Safety Board also commends the FAA for assisting industry implementation of similar systems, such as Boeing has proposed for the 747 center wing fuel tank. The Board appreciates the opportunity to comment on this proposed special condition.

From: NTSB
To: FAA
Date: 3/11/2003
Response: NMC# 101961: The FAA's last correspondence on these recommendations was dated May 17, 2000. On August 23, 2000, the Safety Board issued Aviation Accident Report: In-flight Breakup Over the Atlantic Ocean Trans World Airlines Flight 800 Boeing 747-141, N93119 near East Moriches, New York, July 17, 1996, AAR-00-03, the final report on the TWA 800 accident. In that report, the Board commented on the FAA's activities in response to these recommendations, noting with respect to Safety Recommendation A-96-175: In addition, the FAA indicated that it is evaluating the use of ground sources for conditioned air (instead of using the air conditioning packs) as an additional measure to reduce the flammability of CWTs in existing designs and the current fleet. According to the FAA, use of ground sources on days when the temperature exceeds 60° F would reduce the fleetwide flammability of CWTs from about 35 percent to about 25 percent. The FAA stated that a Boeing service letter (SL) recommends that operators use ground sources for conditioned air when available and practical and that it intends to encourage operators to follow this recommendation. The Safety Board is pleased by the significant reduction in flammability that can be achieved by using ground sources for conditioned air and notes that a requirement that such sources be used seems to be a logical step toward satisfying the intent of Safety Recommendation A-96-175. In the TWA 800 final report, the Board summarized its evaluation and classified these recommendations: Although the use of ground sources for conditioned air can provide meaningful short-term benefits, fuel tank inerting appears to be a more promising, near-term method that could even more dramatically reduce fuel tank flammability in the existing fleet. Thus, the Safety Board is pleased that the FAA has recently begun to aggressively study this method…. The Board strongly encourages the FAA to consider a broader range of inerting technologies and expedite the pace of its research and rulemaking initiatives with regard to fuel tank inerting. However, based on the FAA's recent input to the Board, and pending additional expedited action with regard to fuel tank inerting, Safety Recommendations A-96-174 and -175 are classified OPEN - - ACCEPTABLE RESPONSE. Safety Board staff has been briefed by and has remained in regular contact with FAA staff regarding the promising results of the FAA's research and development activities related to fuel tank inerting, which is responsive to the intent of A-96-174. While implementation has great promise, it may require years of effort and may not be possible in some aircraft. The Safety Board is not aware of any activity that the FAA is taking related to requiring the use of ground sources for conditioned air or other activity to meet the intent of Safety Recommendation A-96-175. The Board notes that Safety Recommendation A-96-175 is an urgent recommendation, although it is now over 6 years old. The Safety Board would appreciate receiving an update from the FAA on current, planned, and completed activities being taken in response to Safety Recommendations A-96-174 and -175.

From: NTSB
To: FAA
Date: 8/23/2000
Response: A complete history of these recommendations is addressed in the Board's accident report adopted 8/23/00. The report is titled "Aaircraft Accident Report--In-Flight Breakup Over the Atlantic Ocean Trans World Airlines Flight 800 Boeing 747-131, N93119 near East Moriches, NY, 7/17/96."

From: NTSB
To: FAA
Date: 8/3/2000
Response: The Safety Board is pleased that the FAA has recently begun to aggressively study this method. However, given that 3 1/2 years have passed since the Board recommended that the FAA give significant consideration to the issue of fuel tank inerting as a means of precluding the operation of transport-category airplanes with flammable vapors in the fuel tanks, and given the nature of the results that have been achieved by prior ARACs (particularly the FTHWG) 655 the Board is concerned that the FAA has chosen to address this issue by forming another ARAC working group to review and advise the FAA regarding the practicality of fuel tank inerting methods. The Board strongly encourages the FAA to consider a broader range of inerting technologies 656 and expedite the pace of its research and rulemaking initiatives with regard to fuel tank inerting. However, based on the FAA™s recent input to the Board, and pending additional expedited action with regard to fuel tank inerting, Safety Recommendations A-96-174 and -175 are classified "Open--Acceptable Response."

From: FAA
To: NTSB
Date: 5/17/2000
Response: Letter Mail Controlled 05/18/2000 3:19:52 PM MC# 2000639: - From Jane F. Garvey, Administrator: The Federal Aviation Administration (FAA) has continued to make progress in addressing action that could significantly reduce the exposure of transport airplane fuel tanks to flammable vapors in both newly certificated and existing airplanes. The methods under evaluation are cooling of the center wing fuel tanks using directed ventilation and fuel tank inerting using onboard nitrogen or ground-based nitrogen sources. In addition to the evaluation of these long-term flammability reduction methods, the FAA proposed a short-term regulation that would require minimizing the flammability of fuel tanks on new transport airplane designs. Short-Term Flammability Regulation: On October 29, 1999, the FAA proposed a new regulation that would require minimizing the flammability of fuel tanks on new transport airplane designs. This was part of the notice of proposed rulemaking (NPRM) that included the proposed Special Federal Aviation Regulation (SFAR) discussed in response to Safety Recommendation A-98-36. As stated in the NPRM and acknowledged by the Board in its comments to the docket, the proposed flammability regulation is not intended to prevent the development of flammable vapors in fuel tanks because methods that could totally prevent flammable vapors have not been found to be practical. The proposed regulation is intended as a short-term measure to preclude, in new designs, the use of designs that result in a relatively high likelihood that flammable vapors will develop in fuel tanks. The intent of the proposal as stated in the NPRM is to require that fuel tanks are not heated, and that they cool to below the ignition temperature at a rate equivalent to that of a wing tank in the transport airplane being evaluated. Since the issuance of the NPRM, the FAA issued a notice requesting comments on draft Advisory Circular (AC) 25.981-2X, Fuel Tank Flammability Minimization, that would provide guidance on acceptable methods of complying with the proposed fuel tank flammability rule. The notice for the draft AC was issued on February 2, 2000, and a copy of the draft AC was provided to the Board's staff. The comment period for both the NPRM and the draft AC closed on March 27, 2000. The FAA is reviewing the comments received on the NPRM and the draft AC. Fuel Tank Inerting: The FAA fuel tank inerting research and development program has continued to make significant progress in the study of fuel tank inerting. The FAA team formed to determine the cost of the ground equipment for ground-based inerting has completed its study, and the report is in final preparation. This study brought together a team of experts that developed system requirements and estimated costs of the ground equipment that would be required to incorporate ground-based inerting. The project team was led by the FAA and included representatives from a nitrogen generating system manufacturer, a large and small airport, airport fuel suppliers, and a large airline. The study did not evaluate the airplane modifications that would be required to incorporate ground-based inerting, but did evaluate the ground-based equipment and the infrastructure required. Data from the report indicate that the cost to implement ground-based inerting is significantly less than the estimate in the 1998 Fuel Tank Harmonization Working Group (FTHWG) report. The FAA expects to issue the ground-based inerting report by the end of June 2000. The FAA has also developed analytical tools to evaluate the effectiveness of ground-based inerting. The FAA analysis using these tools indicates that using ground-based inerting to inert center wing tanks (CWT) located above a!--- conditioning packs would reduce the fleet average flammability exposure for those CWT's to approximately 2 percent. This is about the same benefit predicted in the July 1998 report of the Aviation Rulemaking and Advisory Committee (ARAC) FTHWG. The FAA has completed plans for a ground and flight test program that will provide data on the effectiveness of ground-based inerting. Boeing is supporting this program by providing a Boeing 737 airplane. A nitrogen gas distribution manifold will be installed in the CWT. Instrumentation will be installed to measure fuel tank temperature and the oxygen content in the CWT ullage (vapor space). Ground-based nitrogen generating equipment will be used to inert the CWT on the ground through a connection to the CWT distribution manifold. Ground testing and flight testing will be performed to determine how long the oxygen content in the CWT remains at a level sufficiently low as to be incapable of supporting combustion. These tests will provide valuable data to determine if any airplane vent system modifications are required for ground-based inerting to be effective. The ground and flight testing is expected to be completed by the end of June 2000. With the cost and infrastructure data being developed and the effectiveness model completed, the additional data are still necessary for a decision on implementation in order to determine the types and feasibility of aircraft modifications, and approval bases for existing, newly manufactured, and new aircraft designs. To gather these types of data effectively, the FAA will use the ARAC process since it brings together all necessary industry elements, suppliers of inerting systems, airline operations and maintenance personnel, and aircraft 4 manufacturers. Consequently, the FAA has prepared a tasking statement to establish the new ARAC fuel tank inerting task described in the FAA's letter to the Board dated November 3, 1999. The task will establish a new ARAC working group that will perform a detailed study of ground-based inerting and on-board inerting methods for in-service, newly manufactured, and newly designed transport airplanes. The new ARAC task will develop further data required for the FAA to determine if a method of inerting fuel tanks could be practical for these airplanes. The task will require that the ARAC complete the study within 12 months after the tasking statement is published in the Federal Register. The tasking statement has been coordinated with the Joint Airworthiness Authorities, and the FAA expects to publish the task in the Federal Register by the end of June 2000. Directed Ventilation: Directed ventilation would reduce the heating of center wing fuel tanks on those airplanes that have air conditioning packs located below the CWT and, therefore, reduce the exposure of the CWT to flammable vapors. The heat released from both the packs and the bleed air ducts that supply hot pressurized air to the packs result in heating the CWT. The hot air is supplied from engine compressor sections during flight and from auxiliary power units or ground service equipment when the engines are shut down between flights. Directed ventilation concepts under evaluation would reduce the heating of those fuel tanks by using forced ventilation to remove heat from the pack bay area. Insulation or heat shields would also be incorporated to reduce heating of the tanks with some of the directed ventilation concepts under evaluation. In February 1999, the FAA requested specific fuel tank temperature data from manufacturers of airplanes that have air conditioning packs located below the CWT. The request identified specific flight and ground operations that should be included in the information provided to the FAA. Airbus airplane models and most Boeing airplane models have air conditioning packs located below the CWT. The pack bay areas on Airbus airplanes have a ventilation system installed to keep certain composite structural materials in the pack bay from becoming too hot--not to reduce the heating of the CWT. Boeing has performed design studies to determine what type of directed ventilation systems could be installed in its airplane models and the effect those systems would have in reducing the heating of the CWT. Airbus and Boeing have provided part of these data requested for this study. These data were provided in the form of temperature data produced from analytical models. The models were based on flight test temperature data that where available and heat transfer analysis. These initial data indicate that directed ventilation systems would not reduce the temperature of heated CWT's as much as previously expected. Therefore, it appears that the reduction in the exposure of heated CWT's to flammable vapors provided by directed ventilation based on this preliminary analysis of partial data would not be as significant as predicted in the July 1998 report issued by the ARAC FTHWG. The remaining data requested from the manufacturers are needed before reaching a final conclusion on the benefits of directed ventilation. The FAA will provide an update to the Board on the directed ventilation study when the remaining data have been received and the evaluation completed. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: FAA
To: NTSB
Date: 5/6/2000
Response: Letter Mail Controlled 05/12/2000 3:05:14 PM MC# 2000612:

From: NTSB
To: FAA
Date: 3/24/2000
Response: Notation 7250: The National Transportation Safety Board has reviewed the Federal Aviation Administration’s (FAA) Notice of Proposed Rulemaking (NPRM), “Transport Airplane Fuel Tank System Design Review, Flammability Reduction, and Maintenance and Inspection Requirements,” which was published in 64 Federal Register 58644 on October 29, 1999. The NPRM indicates that the July 17, 1996, accident involving TWA flight 800 and the ensuing Safety Board Safety Recommendations (A-96-174 through -177 and A-98-34 through -39) have prompted the FAA to examine the underlying safety issues surrounding fuel tank explosions, the adequacy of the existing regulations, the service history of airplanes certificated to these regulations, and existing fuel system maintenance practices. The FAA proposes to amend the current regulations to address prevention of ignition sources and minimization of flammable vapors in fuel tanks in future airplane designs. The Board generally supports the proposed regulatory changes; however, as discussed below, the Board believes that the FAA needs to give further consideration to some associated issues. Prevention of Ignition Sources in Fuel Tanks The NPRM proposes a Special Federal Aviation Regulation (SFAR) that would require type certificate (TC) holders for transport airplanes and holders of supplemental type certificates (STC) that affect the airplane’s fuel system to conduct a safety review of the fuel tank system that is designed to show that fuel tank fires or explosions will not occur. Specifically, the TC or STC holder would be required to determine whether the design meets the existing requirements of 14 Code of Federal Regulations (CFR) Section 25.901 and the requirements of Section 25.981(a) and (b), which would include several provisions added by the NPRM. If the design does not meet these requirements, the SFAR would require the TC or STC holder to develop the necessary corrective design changes. Section 25.981 currently requires that “a safe margin” exist between the temperature at any place inside a fuel tank where fuel ignition is possible and the lowest expected autoignition temperature of the fuel in the fuel tank. The NPRM proposes to revise Section 25.981 so that subsection (a) would prohibit an ignition source from being present at any point in the fuel tank system “where catastrophic failure could occur due to ignition of fuel or vapors.” The new rule would require that this be shown by demonstrating (1) compliance with the existing requirement regarding autoignition temperature and (2) that an ignition source in the fuel tank system could not result from any single failure, from any single failure in combination with any latent failure condition not shown to be extremely remote, or from any combination of failures not shown to be extremely improbable. The revised Section 25.981 would also require, in subsection (b), that critical design configuration control limitations, inspections, or other procedures be established as necessary to prevent development of ignition sources. According to the NPRM, the design approval holder would be expected to do the following to comply with the SFAR: develop a failure modes and effects analysis (FMEA) for all components in the fuel tank system. Analysis of the FMEA would then be used to determine whether single failures, alone or in combination with foreseeable latent failures, could cause an ignition source to exist in a fuel tank. A subsequent quantitative fault tree analysis should then be developed to determine whether combinations of failures expected to occur in the life of the affected fleet could cause an ignition source to exist in a fuel system. The Safety Board identified numerous potential ignition hazards (including, but not limited to, aging components, contamination and corrosion of components, and sulfide deposits on components) during the TWA flight 800 investigation. The FAA states that its intention is that these failure conditions, and any other foreseeable failures, should be assumed when performing the FMEA analysis. The Safety Board generally supports the intent of the SFAR in requiring an FMEA analysis, but is concerned about the construction of the FMEAs, as well as the thoroughness and integrity of the data that will be used. Concerns about the construction and integrity of data were raised during a recent review of a proprietary fault tree analysis that a manufacturer developed in response to a Board request as part of the TWA flight 800 accident investigation. The fault tree analysis described various potential failures and combinations of events that could lead to the ignition of the center wing fuel tank. Because of concerns that arose during a review by the Safety Board of the original fault tree analysis and a subsequent revision, the Board requested that National Aeronautics and Space Administration (NASA) failure analysis specialists examine the document. A November 25, 1998, letter from Ms. Amanda H. Goodson, NASA’s Director for Safety and Mission Assurance, summarized the NASA review as follows: Many of the probabilities, failure rates, and/or exposure times were much lower than would reasonably be expected. The probability of occurrence should be higher and/or exposure times should be longer on many of the basic events….Based on our evaluation of the tree and the information provided by the NTSB, the subject fault tree analysis quantification cannot stand up to peer review and should not be viewed as realistic. It should be noted that the logic of the tree could not be fully evaluated since we did not have access to the engineering drawings and schematics of the system. However, based on previous systems experience, we would expect the tree to be constructed differently. The Safety Board’s concerns about the FMEAs are amplified by the fact that no single source exists for reliable and comprehensive data on component failures or malfunctions. Because the calculations in a FMEA are based on failure rates, incomplete or inappropriate failure data can skew the results of an examination. The Board is aware that service history data maintained by manufacturers do not capture data from all operators. Further, the Board has found that the amount of data provided by the manufacturers of replacement component parts sometimes greatly exceeds the data provided by the aircraft manufacturers (possibly because replacement parts suppliers can sell parts directly to operators and repair facilities). Although the FAA collects a significant amount of data about mechanical failures through its Service Difficulty Report (SDR) program, even these data are incomplete. Other sources of potentially relevant data are the service histories maintained by the military of its variants of commercial airliners and the Board’s accident and incident investigation database; however, neither of these sources provides complete data either. Further, the many affected TC and STC holders (some of which are not the original designers or manufacturers) may have varying levels of experience with developing FMEAs. In addition, the Safety Board is concerned that engineers working for TC and STC holders may not recognize the existence or significance of certain hazards and that potentially competing interests may affect the quality and thoroughness of some FMEAs. In cases in which the TC or STC holder no longer exists, FAA personnel with varying levels of skill and experience may have to conduct the FMEAs. Finally, the Safety Board is concerned that the FAA may have an insufficient number of staff who are trained to properly evaluate an FMEA. Therefore, to ensure the integrity and effectiveness of the fuel tank system safety review, the Safety Board urges the FAA to develop and provide adequate standards and criteria to guide the development of the FMEAs and fault tree analyses. In particular, because there is no single comprehensive collection system that contains data on the failure of airplane components and because of the inadequacies that exist in each source of data, those guidelines should specify that the data used for the FMEAs must be collected from all available sources, including operators, manufacturers, and appropriate government agencies. The FAA needs to provide adequate oversight and auditing of the FMEA analysis results to ensure their technical accuracy and integrity. In particular, such oversight and auditing should include a review of the data sources used to ensure that all available and appropriate sources of failure data have been taken into account. FAA oversight and auditing can also provide a method to identify potential deficiencies in the FMEAs that might not be recognized by engineers and designated engineering representatives who work with these systems on a daily basis. The SFAR would also require TC and STC holders to develop all maintenance and inspection instructions necessary to maintain the design features required to preclude the existence or development of an ignition source within the fuel tank system. The Safety Board strongly endorses continuing airworthiness through improvements to maintenance, inspection, and minimum equipment lists. However, given the very general nature of some current inspection criteria pertaining to fuel tank safety (as documented by the FAA in its Transport Non-Structural Systems Plan), the Board is concerned that the instructions resulting from the SFAR requirement may be similarly broad and, therefore, potentially ineffective. Many potential ignition sources (such as hidden cracks in wiring, sulfide deposits, and use of inappropriate materials) may not be apparent during a general visual inspection. Therefore, the Board urges the FAA to ensure that the maintenance and inspection instructions developed as a result of this SFAR are detailed and specific enough to provide mechanics with useful inspection criteria and to ensure that they are properly trained about how to effectively carry out those instructions, including a requirement for a detailed inspection of each component in any area that may be exposed to fuel or fuel vapors. Minimizing Development of Flammable Vapors in Fuel Tanks The NPRM also proposes to add a new subsection (c) to 14 CFR Section 25.981, which would require that fuel tank installations in newly designed airplanes include a means to minimize the development of flammable vapors in fuel tanks, or to mitigate the effects of an ignition of fuel vapors within the fuel tanks, such that no damage caused by an ignition will prevent continued safe flight and landing. (Examples of means by which such mitigation could be accomplished, and which are being actively studied, are installation of fire suppressing polyurethane foam to extinguish or retard ignition of fuel vapor and installation of explosion suppression systems. The Safety Board notes that there are numerous unresolved operational and maintenance problems inherent in such in-tank mitigation technologies. In light of the FAA’s limited resources, the Safety Board urges the FAA to attempt to realize more immediate and effective safety improvements by focusing its resources on methods for minimizing the development of flammable vapors, rather than means for mitigating the effects of ignition.) The FAA acknowledges that this proposal is not intended to prevent the development of flammable vapors because total prevention has not been found to be feasible. Rather, the FAA states that the proposal is intended as an interim measure to preclude, in new designs, the use of design methods that result in a relatively high likelihood that flammable vapors will develop in fuel tanks. The Safety Board is pleased that the FAA has recognized that minimizing the development of flammable fuel vapors in fuel tanks is necessary to reduce the risk of fuel tank explosions and supports the proposed changes to 14 CFR Section 25.981. Further, the Board understands that this is an interim measure and looks forward to receiving further information from the FAA once it completes its evaluation of and research into means for minimizing the development of flammable vapors within fuel tanks and develops a definitive standard to address this issue in new designs. However, the Safety Board is concerned that the NPRM does not propose any regulatory changes that address fuel tank flammability in current designs and in the existing fleet. This is especially disturbing because some operational measures (such as limiting the on-ground operating time of air conditioning packs and substituting a ground-based cool air supply and cooling or ventilating the pack bay) that can reduce current levels of flammable vapors could be accomplished immediately. The Board is also aware that the FAA is conducting research into on-ground fuel tank inerting systems for the existing fleet. Because the Board believes that fuel tank inerting is a promising, near-term method that could dramatically reduce fuel tank flammability in the existing fleet, it strongly supports the FAA’s continued work in this regard and looks forward to regulatory implementation. In the NPRM, the FAA discussed the conclusions of the Aviation Rulemaking Advisory Committee’s (ARAC) Fuel Tank Harmonization Working Group (FTHWG), which was established on January 23, 1998, to evaluate methods to reduce or eliminate hazards associated with explosive vapors in fuel tanks. The FTHWG concluded that the safety record of fuel tanks located in the wings (which the FTHWG calculated were flammable about 7 percent of the fleet operational time) was adequate and that if the same level of safety could be achieved in center wing fuel tanks the overall safety objective could be achieved. Thus, the FTHWG proposed limiting the airplane’s exposure to flammable conditions in all fuel tanks to less than 7 percent of the expected fleet operational time. Although FAA staff have indicated to Safety Board staff that the FAA does not intend to endorse the FTHWG’s proposed exposure criteria, the Board nonetheless wishes to register its concerns about those criteria. Because it is a fleetwide average, it does not account for increased risks that may exist at specific locations, during certain time periods, or for certain flights. In addition, the premise that transport airplane fuel tanks located in the wings have an acceptable safety record is unacceptable because wing fuel tanks have exploded. The Safety Board believes that the goal should be to completely eliminate the development of flammable vapors in fuel tanks to the greatest extent technically feasible (such as would result from the use of on-ground inerting systems). The Safety Board appreciates the opportunity to comment on this proposed rulemaking.

From: FAA
To: NTSB
Date: 11/3/1999
Response: Letter Mail Controlled 11/05/1999 12:48:36 PM MC# 991232: - From Jane F. Garvey, Administrator: The Federal Aviation Administration (FAA) has followed the action plan described in its letter dated December 3, 1997, and on January 23, 1998, published a notice in the Federal Register to establish an Aviation Rulemaking Advisory Committee (ARAC) Fuel Tank Harmonization Working Group on fuel tank flammability reduction. The notice gave the working group 6 months to provide the FAA with a report describing all technical issues and specific solutions for implementing improvements in transport airplane fuel tank flammability issues. The ARAC working group report was issued and transmitted to the FAA on July 23, 1998. A CD-ROM electronic copy of the report was been provided to the Board. The report is also available on Internet through the FAA Technical Center, Fire and Cabin Safety web site, at http://www.fire.tc.faa.gov (select "REPORTS" from this web site). The ARAC working group evaluated the service history of transport airplane fuel tank fire and explosion events that have occurred since 1959. It evaluated fuel tank flammability exposure among various airplane types and concluded that wing tanks and unheated center wing tanks currently have a low exposure to flammable vapors. The report further states that the ARAC working group concluded that reducing flammability in all fuel tanks to the level of the wing tanks on most airplanes was seen as a worthwhile goal. The working group examined a large variety of design concepts to determine if they were technically and economically feasible for retrofit of the existing fleet, incorporation into new production airplanes, or use in new airplane designs. The working group concluded that none of those concepts were currently feasible for retrofit on in-service transport airplanes, but that changes could be made early in the design process for a new airplane to reduce the flammability of center fuel tanks. The following are the conclusions that are contained in the ARAC Fuel Tank Harmonization Working Group Report: • Techniques to reduce or eliminate heat input to the tanks from nearby heat sources were evaluated. Of these techniques, directed ventilation and relocation of the significant heat sources reduce the exposure to an acceptable level. However, relocation is only feasible for new airplane designs. Directed ventilation for in-service aircraft is estimated to have an overall cost for a 10-year period of $3.5 billion. • To reach the goal by changing fuel properties, a minimum flash point specification of 140 degrees Fahrenheit would be required. A change of this magnitude falls outside the current experience base and may require engine redesign/requalification. The overall fuel manufacturing cost increase for a 10-year period is estimated at $15 billion in the United States and $60 billion for the rest of the world and could result in a significant shortfall of jet fuel. • Techniques such as on-board fuel tank inerting or installation of foam in the tanks would also achieve the goal, but at an estimated cost estimated of $20 billion over the next 10 years and would be very difficult to retrofit in current airplanes. Ground inerting, wherein specific tanks are made inert prior to flight, at specific airports is an option that needs future study to determine the logistical costs of such a system and if retrofit installation of the distribution system internal to the airplane could be achieved in a cost-effective manner. • The Working Group considered several concepts that were determined to be insufficiently advanced technically at this time for transport airplane fuel tank use. These included ullage sweeping and explosion suppression systems. The ARAC report also made the following specific recommendations to the FAA: 1. Adopt the proposed new regulatory action on new aircraft designs. 2. Continue to investigate means to achieve a cost-effective reduction in flammability exposure for the in-service fleet and newly manufactured aircraft. 3. Pursue the studies associated with directed ventilation and ground-based inerting systems to improve their cost effectiveness. 4. If a practical means of achieving a cost-effective reduction in flammability exposure can be found for the in-service fleet, either at the level specified in the rule or at some intermediate level (recommendations 2 and 3 above), consider application of that solution in combination with other actions (e.g., SFAR). 5. If a practical means of achieving a cost-effective reduction in flammability exposure can be found for newly manufactured aircraft, either at the level specified in therule or at some intermediate level (recommendations 2 and 3 above), consider application of that solution, in combination with other actions (e.g., SFAR). The report reflects a significant effort by the ARAC working group to organize as a group, compile and analyze a large amount of information, and produce a final report in only 6 months. The FAA has reviewed the report and evaluated the conclusions and recommendations made by the working group. While the FAA may not agree with everything contained in the report, it does agree with the conclusion that further study is needed to determine if a method can be shown to be feasible that would eliminate or reduce the flammability of fuel tanks on in-service transport airplanes. The FAA has, therefore, initiated studies of the directed ventilation and ground-based inerting methods that are described in the ARAC report. This further evaluation is intended to develop the detailed information necessary for the FAA to determine if regulations that would require retrofit of such modifications on in-service and new production transport airplanes could be feasible. The FAA studies include evaluation of the effectiveness of ventilation methods currently used on Airbus airplanes and possible application of this technology to other airplane models with center wing fuel tanks near heat sources. Review of the ventilation means at a meeting with Airbus indicates that significant heat is transferred from the air conditioning bay located below the center wing tank. However, detailed temperature data are not currently available so that overall effectiveness can be determined. Airbus and Boeing provided the FAA with initial fuel tank temperature data, and the completed package will be available by the end of March 2000. The FAA has also initiated a research and development project to determine equipment requirements for ground-based inerting that may be feasible for use on in-service and current production airplanes. This project will bring together the experts necessary to develop system requirements, including inerting system hardware manufacturers, airports, airline operators, and airplane manufacturers. The FAA expects the group's initial findings to be available by the end of December 1999. After receipt of the group's initial findings, the FAA plans to task the ARAC to perform a detailed study of ground-based inerting and on-board inerting options. This new ARAC study would include designs for retrofit of existing airplanes, incorporation into new production airplanes, and incorporation into new designs. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: NTSB
To: FAA
Date: 9/21/1999
Response: The Safety Board is concerned that in the 2 1/2 years since this recommendation was issued no design modifications to airplanes have been made, such as nitrogen-inerting systems or the addition of insulation between heat-generating equipment and fuel tanks. The Safety Board believes that such actions would reduce the probability of fuel/air explosions in fuel tanks. Because the FAA has taken no action towards implementing A-96-174, and it appears that none is contemplated, it is classified OPEN – UNACCEPTABLE RESPONSE. The Safety Board requests that the FAA update its position regarding this recommendation.

From: FAA
To: NTSB
Date: 3/3/1998
Response: Letter Mail Controlled 3/5/98 4:04:27 PM MC# 980293 - From Jane F. Garvey, Administrator: The FAA is following the action plan described in its letter dated 12/3/97, to establish an aviation rulemaking advisory committee (ARAC) working group on fuel tank flammability reduction. The ARAC executive committee met on 1/8/98, to review the task assignment for the new working group. The ARAC executive committee has accepted the task assignment, and the FAA has prepared the notice of the new task assignment to establish the ARAC working group. This notice was signed on 1/20/98, and published in the Federal Register on 1/23/98. This notice will give the ARAC working group until 7/23/98, to provide the FAA with a report describing all technical issues and specific solutions for implementing improvements in transport airplane fuel tank safety issues.

From: FAA
To: NTSB
Date: 12/3/1997
Response: Letter Mail Controlled 3/18/98 4:46:08 PM MC# 971618: - From Jane F. Garvey, Administrator: As a result of the comments received from our April 3 notice and the recent Fuel Flammability Conference, the FAA has concluded that improvements can be made that will greatly reduce, if not eliminate, the possibility of fuel tank explosions. The FAA plans the following actions: * Along with FAA's continuing efforts to make improvements, the Aviation Rulemaking Advisory Committee (ARAC) will be directed to identify specific methods to implement improvements in transport airplane fuel tank safety for the current and future fleet. This project will be on a fast-track basis. After the Notice of the New Task Assignment goes to the Federal Register, the ARAC working group will be given 6 months to provide the FAA with a technical report describing all technical issues and specific solutions. The ARAC will, using the data presented in response to the notice dated April 3, 1997, and any further information which may be developed from the Board's ongoing investigative efforts, evaluate both reducing or eliminating explosive fuel-air vapors within airplane fuel tanks and a further reduction in the potential of ignition sources within fuel tanks. FAA will then act upon the ARAC recommendations and make appropriate judgment and decisions on further action expeditously. * As you know, the ARAC is composed of industry experts, as well as a number of passenger, union, and public interest group members. Comments to the notice indicate that many methods of reducing exposure to explosive fuel-air mixtures exist. The methods include development of aviation fuels with modified properties; airplane modifications for controlling fuel temperatures; fuel tank inerting using gases such as nitrogen, carbon dioxide, or engine exhaust gases; explosion suppression systems; and polyurethane-reticulated foam. The methods provide varying degrees of reduction in the time the airplane fuel tanks would be operated with explosive fuel-air mixtures in the fuel tank ullage spaces. The associated costs, benefits, and weights also vary. The FAA believes the best method of assimilating this needed information is within an ARAC working group. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: NTSB
To: FAA
Date: 7/1/1997
Response: The Safety Board recognizes the existence of technological advances, such as permeable membrane nitrogen inerting systems, developed for current military aircraft. The Board is also aware that because the chemical industry had been unable to prevent every source of ignition energy from gaining access to flammable gases, it decided to inert many types of chemical storage tanks. The Board believes that the type of fundamental regulatory changes requested in A-96-174 are the type of changes called for by the White House Commission on Aviation Safety and Security, under the direction of V.P. Albert Gore. Although the Board is concerned about the length of time it took for the FAA to publish the request for comments, the Board recognizes the value in the FAA seeking and evaluating input from the industry before mandating policy change that could require significant modifications to previously approved air carrier aircraft designs. Accordingly, A-97-174 and -177 are classified OPEN – ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 6/27/1997
Response: MC# 970823: - From Barry L. Valentine, Acting Administrator: The Federal Aviation Administration's (FAA) senior technical staff met with the Board's staff to address the critical technical issues raised in these safety recommendations. There is no question that the FAA shares with the Board the goal of minimizing the risk of fuel tank explosions and the belief that the reduction of ignition sources and the reduction of fuel volatility are the most promising dual paths to this objective. The FAA believes there was technical agreement in several important areas. There was agreement that more research is necessary to understand fully what happened within the center fuel tank to cause it to explode. The FAA agreed that the research needs to focus on the chemistry of fuel vapors and the energy of the ignition sources necessary to ignite those vapors. That research is already underway by the Board, through a contract with the California Institute of Technology. The FAA fully supports this effort and is willing to co-manage the program to maximize its cooperation in this very important research. A second area of agreement is the need to conduct more flight tests to understand fully the conditions in the center fuel tank and vent system that existed at the time of the accident. In early July 1997, the Board will conduct flight tests using an instrumented Boeing 747-100 airplane. These tests will provide critical data about fuel and vapor temperatures in the center wing tank, as well as other valuable information, which are necessary to understanding the difficult technical issues related to this accident. The scope and objectives of this initiative were subjects of the recent meetings of our respective staffs, and there was agreement that the outcomes of the flight tests will be critical to the understanding and resolution of these technical questions. In discussions with your staff, the FAA believes there was agreement that this flight test as well as the California Institute of Technology work are required to determine the efficacy of the safety recommendations. I believe that upon completion of these flight tests, the Board, the FAA, and the industry will have a more complete understanding of the next steps that must be taken on this critical issue. As agreed with your staff, the FAA will use these data as a foundation for the safety measures required to resolve these issues. The FAA will continue to work closely with your staff in the investigation and on the various technical activities. The FAA agrees with the Board that this scientific analysis and other tests are needed to determine the best actions to be taken with respect to the TWA 800 issues. Along with the other Board efforts that the FAA supports fully, the FAA has also asked the aviation and scientific community to provide information on research and other data related to the fuel tank issues that are raised by the Board's recommendations. Both the FAA and the Board agree that any measures taken to address these safety concerns cannot produce uncertain safety benefits, which may in fact decrease safety. The FAA also believes that the Board's staff agreed with the need to pursue other avenues of explosion prevention. The FAA has an aggressive program to pursue a further minimization of ignition sources within the fuel tanks. Such things as ground fault protection of wires in fuel tanks and more focused maintenance programs for all fuel tank components are examples of several avenues the FAA is pursuing.

From: FAA
To: NTSB
Date: 2/18/1997
Response: MC# 970179: - From Barry L. Valentine, Acting Administrator: The FAA response states that the airworthiness standards of 14 CFR Part 25 assume that fuel vapor is flammable, that current fuel tank design requirements dictate elimination of ignition sources, and that these safety recommendations propose major changes in the requirements for fuel tank design and fuel management in transport category airplanes. The FAA stated that fuel management to control the temperature [of the ullage] in airplane fuel tanks may have little benefit, and controlling temperature to the extent necessary to ensure that fuel tank vapor is nonflammable may be a task with significant economic implications. Instead of responding directly to the recommendations or initiating research into how the recommendations might be implemented, the FAA stated that it would publish, within 30 days, a notice asking for public comment on the effectiveness and feasibility of the Board's recommendations was published in the Federal Register on April 3, 1997.

From: NTSB
To: FAA
Date:
Response: At the 1997 Board meeting addressing the NTSB’s Most Wanted List of Transportation Safety Improvements (MWL), the Board voted to place Safety Recommendations A-96-174 through A-96-176 on the Federal MWL under the issue category “Explosive Mixtures in Fuel Tanks.” In 1997 the category's named changed to "Flammable Fuel/Air Mixture in Fuel Tanks on Transport Category Aircraft." This issue category was removed from the MWL in 2008.