U.S. Department

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: FUEL TANK FLAMMABILITY REDUCTION MEANS Date: 9/19/08 AC No. 25.981-2A Initiated by: ANM-112 1. Purpose. This advisory circular (AC) provides information and guidance on compliance with the airworthiness standards for transport category airplanes about limiting the time a fuel tank may be flammable or mitigation of hazards from flammable fuel air mixtures within fuel tanks. This guidance is applicable to transport category airplanes for which a new, amended, or supplemental type certificate is requested, and for which Amendment 25-125 applies. It is also applicable to certain existing design approval holders and certain pending applications for new type certificates, supplemental type certificates and amended type certificates where required by §§ 26.33, 26.35, 26.37, and 26.39, contained in a subpart D to Title 14, Code of Federal Regulations (CFR) part 26, "Fuel Tank Flammability." Guidance on compliance with the associated requirements for operators of affected airplanes that must comply with requirements in 14 CFR parts 121, 125 and 129, to incorporate flammability reduction or ignition mitigation means by specified dates, will be contained in a separate document. 2. Applicability. a. This guidance provided in this document is for design approval applicants and holders, airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) transport category airplane type certification engineers and their designees. b. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. It describes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. The FAA will consider other methods of demonstrating compliance that an applicant may elect to present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in demonstrating compliance with the relevant regulations. On the other hand, if we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance. EFTA00796624 9/19/08 AC 25.98I-2A c. This material does not change, create any additional, authorize changes in, or permit deviations from, regulatory requirements. 3. Cancellation. Advisory Circular (AC) 25.981-2, Fuel Tank Flammability Minimization, dated 4/18/01, is cancelled. 4. Related Documents. a. Federal Aviation Regulations. The applicable sections of part 25 that prescribe the design requirements for the substantiation and certification about prevention of ignition sources within the fuel tanks of transport category airplanes include: § 25.863 Flammable fluid fire protection. § 25.901 Installation. § 25.954 Fuel system lightning protection. § 25.981 Fuel tank ignition prevention. b. Advisory Circulars (AC). You can get the following FAA ACs from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75'h Avenue, Landover, MD 20785, or on the internet at: http://www.airweb.faa.gov/rgl. (1) AC 25-8 Auxiliary Fuel System Installations. (2) AC 20-53B Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Caused by Lightning. (3) AC 25.981-1B Fuel Tank Ignition Source Prevention Guidelines. (4) AC 120-27 Aircraft Weight and Balance Control. (5) AC 26-1 Part 26, Continued Airworthiness and Safety Improvements (6) AC 25-26 Development of Standard Wiring Practices Documentation c. Society of Automotive Engineers (SAE) Documents. You can get the following documents from the Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, Pennsylvania, 15096. (1) SAE AIR 5128, "Electrical Bonding of Aircraft Fuel System Plumbing Systems" (January 1997). (2) SAE AIR 4170A, "Reticulated Polyurethane Safety Foam Explosion Suppressant Material for Fuel Systems and Dry Bays" (November 1998). 2 EFTA00796625 9/19/08 AC 25.98I-2A (3) SAE AIR 1662, "Minimization of Electrostatic Hazards in Aircraft Fuel Systems" (October 1984). d. Military Specifications (MIL). (1) MIL-B-83054, Baffle and Inerting Material, Aircraft Fuel Tank (March 1984). (Note: this reference provides an extensive list of military specifications about the use of polyurethane foam.) You can get a copy of this document from the Department of Defense, Document Automation and Production Service, Building 4/D, 700 Robbins Avenue, Philadelphia, PA 19111-5094, or on the intemet at http://assist.daps.dla.mil/online/start/. e. Other. (I) FAA Document DOT/FAA/AR-98/26, "A Review of the Flammability Hazard of Jet A Fuel Vapor in Civil Transport Aircraft Fuel Tanks" (June 1998). (You can get a copy of this report from the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.fire.tc.faa.gov/pdf/ar98-26.pdf. (2) Aviation Rulemaking Advisory Committee (ARAC), Fuel Tank Harmonization Working Group, Final Report (July 1998). You can get a copy of this report at the following web site address: http://www.regulations.gov, by inserting the associated docket number (Docket No. FAA-I 998-4183) into the advanced docket search function. You can also get a copy of this report at the following web site address: http://www.fire.tc.faa.gov/systems/fueltank/papers.stm. (3) "Effects of Fuel Slosh and Vibration on the Flammability Hazards of Hydrocarbon Turbine Fuels Within Aircraft Fuel Tanks," Technical Report AFAPL-TR- 70-65 (November 1970), Edwin E. Ott. Contact the Air Force Aero Propulsion Laboratory, Airforce Systems Command, Wright-Patterson Air Force Base Ohio. (4) FAA Document DOT/FAA/AR-99/65, "Mass Loading Effects on Fuel Vapor Concentrations in an Aircraft Fuel Tank Ullage." You can get a copy of this report from the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.fire.tc.faa.gov/pdf/tn99-65.pdf. (5) FAA Document DOT/FAA/AR-00/19, "The Cost of Implementing Ground- Based Fuel Tank Inerting in the Commercial Fleet." DOT/FAA/AR-00/19 (May 2000). You can get a copy of this report from the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.fire.tc.faa.gov/pdf/00-19.pdf. (6) FAA Document DOT/FAA/AR-01/6, "Inerting of a Vented Aircraft Fuel Tank Test Article with Nitrogen Enriched Air" (December 2000). You can get a copy of 3 EFTA00796626 9/19/08 AC 25.98I-2A this report from the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.fire.tc.faa.gov/pdf/01-6.pdf. (7) "The Effectiveness of Ullage Nitrogen-Inerting Systems against 30-mm High-Explosive Incendiary Projectiles," China Lake Naval Weapons Center, J. Hardy Tyson and John F. Barnes, May 1991. You can get a copy of this report at the following web site address: httn://www.reaulations.gov by inserting the docket number associated with this AC (FAA-2005-22997) into the advanced docket search function. (8) FAA Document DOT/FAA/AR-TN02/79, "Limiting Oxygen Concentrations Required to Inert Jet Fuel Vapors Existing at Reduced Fuel Tank Pressures" (April 2003). You can get a copy of this report from the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.fire.tc.faa.gov/pdf/TN02-79.pdf. (9) FAA Special Condition, "Boeing Model 747- 100/2008/200F/200C/SR/SP/100B/300/100B SUD/400/400D/400F Airplanes; Flammability Reduction Means (Fuel Tank Inerting)," Docket No. NM270; Special Conditions No. 25-285-SC. You can get a copy of these special conditions at the following web site address: http://www.regulations.gov, by inserting the docket number associated with this AC (FAA-2005-22997) into the advanced docket search function. (10) FAA Document DOT/FAA/AR-07/30, "Jet A Volatility Survey," July 2007. A copy of this report is available through the National Technical Information Service (NTIS), Springfield, Virginia 22161, or at the following web site address: http://www.tc.faa.gov/its/worldpac/techrpt/ar0730.pdf. (II) FAA Document DOT/FAA/AR-05/8, "Fuel Tank Flammability Assessment Method User's Manual" (May 2008), web site address: http://www.fire.tc.faa.gov/systems/fueltank/FTFAM.stm (12) FAA Document DOT/FAA/AR-04/41, "Evaluation of Fuel Tank Flammability and the FAA Inerting System on the NASA 747 SCA" (December 2004), web site address: http://www.fire.tc.faa.gov/pdf/04-41.pdf. (13) Aviation Rulemaking Advisory Committee (ARAC), Fuel Tank Inerting Harmonization Working Group, Final Report (February 2002). You can get a copy of this report at the following web site address: http://www.regulations.gov by inserting the associated docket number (U.S. Department of Transportation (DOT) electronic dockets, Docket No. FAA-2005-22997) into the advanced docket search function. You can also get a copy of this report, at the following web site address: http://www.faa.gov/regulations_policies/rulemaking/cornmittees/arac/media/ec/EC_FT_ T2.pdf (14) FAA Order 8110.104, Responsibilities and Requirements for Implementing Part 26 Safety Initiatives, Effective Date 12/3/07, You can get FAA Orders from the 4 EFTA00796627 9/19/08 AC 25.98I-2A U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785, or available on the Internet at: http://www.ainveb.faa.gov/rgl. (15) FAA Memo No. ANM-08-113-001, Policy Statement on Clarification of Maximum Payload Capacity Definition in Design Approval Holder Rules, dated September 12, 2008. You can get a copy on the interne at: http://www.ainveb.faa.gov/rgl. 5. Definitions. a. Auxiliary Fuel Tank is a tank installed to make additional fuel available for increasing the flight range of that airplane. The term "auxiliary" means that the tank is secondary to the airplane's main fuel tanks, i.e., the functions of the main tanks are immediately available and operate without immediate supervision by the flightcrew in the event of failure or inadvertent depletion of fuel in an auxiliary tank. Auxiliary tanks are usually intended to be emptied of usable fuel during flight and have been installed in various locations including center wing structure, horizontal stabilizers, wings and cargo compartments. Therefore, auxiliary fuel tanks are "normally emptied" fuel tanks as defined below. b. Main Fuel Tank is defined in § 25.981(b)(3)(iii) as "a fuel tank that feeds fuel directly into one or more engines and holds required fuel reserves continually throughout each flight." The functions of the main tanks are immediately available and operate without immediate supervision by the flightcrew in the event of failure or inadvertent depletion of fuel in an auxiliary tank. Generally, main tanks are those dedicated to the feed of the engines during engine feed isolation. c. Normally Emptied, with respect to fuel tanks, is defined in § 26.31(b) as "a fuel tank other than a Main Fuel Tank." Main Fuel Tank is defined in § 25.981(b), and expanded above. d. Bulk Average Fuel Temperature is defined in paragraph N25.2(a) of appendix N to part 25 as "the average fuel temperature within the fuel tank, or different sections of the tank if the tank is subdivided by baffles or compartments." e. Center Wing Tank (CWT) is a fuel tank located partially or entirely in the center of an airplane's wing box. f. Design Approval Holder (DAH) is defined in AC 26-1 as the holder of any design approval, including type certificate, amended type certificate, supplemental type certificate, amended supplemental type certificate, parts manufacturer approval, Technical Standard Order (TSO) authorization, letter of TSO design approval, and field approvals. The definition in AC 26-1 adds that in particular contexts, the term DAH may also refer to applicants for design approvals. In the context of this AC, the term DAH applies to applicants for new design approvals and for changes to existing designs, and it 5 EFTA00796628 9/19/08 AC 25.98I-2A applies to holders of design approvals and applicants for design approvals affected by part 26, subpart D. g. Flammable, with respect to a fluid or gas, means susceptible to igniting readily or to exploding (14 CFR part 1, Definitions). A non-flammable ullage is one where the fuel-air vapor is too lean or too rich to burn or is inert as defined below. As defined in paragraph N25.2(c) of appendix N to part 25, "a fuel tank that is not inert is considered flammable when the bulk average fuel temperature within the tank is within the flammable range for the fuel type being used. For any fuel tank that is subdivided into sections by baffles or compartments, the tank is considered flammable when the bulk average fuel temperature within any section of the tank, that is not inert, is within the flammable range for the fuel type being used. h. Fleet Average Flammability Exposure is defined in paragraph 25.2(e) of appendix N to part 25 as "the percentage of the flammability exposure evaluation time (FEET) each fuel tank ullage is flammable for a fleet of an airplane type operating over the range of flight lengths in a world-wide range of environmental conditions and fuel properties as defined in this appendix" (appendix N to part 25). Section 25.981(b)(3)(ii) explains this term "means the percent of time each fuel tank ullage is flammable for a fleet of an airplane type operating over the range of flight lengths." Fleet average flammability exposure is the total time the tank is flammable divided by the total FEET. i. Flammability Exposure Evaluation Time (FEET) is defined in paragraph N25.2(b) of appendix N to part 25 as "the time from the start of preparing the airplane for flight, through the flight and landing, until all payload is unloaded and all passengers and crew have disembarked. In the Monte Carlo program, the flight time is randomly selected from the Flight Length Distribution (Table 2), the pre-flight times are provided as a function of the flight time, and the post-flight time is a constant 30 minutes." Table 2 referenced in this definition is Table 2 of appendix N to part 25. j. Flammability Envelope is the pressure (i.e., altitude)/temperature domain where the fuel vapor/air mixture is flammable. This flammability envelope is defined in appendix N to part 25, by the upper flammability limit (UFL) and the lower flammability limit (LFL). These flammability limits are dependent on the type of fuel used and vary for different fuel batches that meet the fuel specification. For Jet-A fuels the variation of flash points that are to be used in the analysis are defined in appendix N to part 25. Appendix N also defines the LFL and UFL as a function of altitude and fuel flash point. The flammability envelope for the fuel is defined by the UFL and LFL as follows: (I) LFL at sea level is the flash point temperature of the fuel at sea level minus 10 degrees F. LFL decreases from sea level value with increasing altitude at a rate of 1 degree F per 808 feet. (2) UFL at sea level is the flash point temperature of the fuel at sea level plus 63.5 degrees F. UFL decreases from the sea level value with increasing altitude at a rate of 1 degree F per 512 feet. 6 EFTA00796629 9/19/08 AC 25.98I-2A k. Flammability Reduction Means (FRM) is any system intended to meet the flammability exposure criteria in appendix M to part 25. I. Flash Point of a flammable fluid is defined in paragraph N25.2(d) of appendix N to part 25 as "the lowest temperature at which the application of a flame to a heated sample causes the vapor to ignite momentarily, or "flash." The flash point of a fuel is determined using the standardized test method(s) permitted by the fuel specification. Table 1 of appendix N to part 25, provides the Gaussian (or "normal") distribution for the flash point of the standard (Jet-A) fuel to be used in the analysis. m. Fuel Types approved for use for a given airplane type are listed in the Airplane Flight Manual (AFM) and the type certificate data sheet. Each fuel type has its own properties; those directly related to flammability are "flash point" and "distillation" characteristics. Property differences can occur in different batches of a given fuel type because of variations in the properties of the source crude oil and the refining process used to produce the fuel. The most widely used fuel types are JET-A or JET-Al, per ASTM International Specification D1655, "Standard Specification for Aviation Turbine Fuels." Older airplanes have been approved for use of JET-B (JP-4), per ASTM Specification D66I5, "Specification for Jet B Wide-Cut Aviation Turbine Fuel." n. Gaussian Distribution is defined in paragraph N25.2(f) of appendix N to part 25 as "another name for the normal distribution, a symmetrical frequency distribution having a precise mathematical formula relating the mean and standard deviation of the samples. Gaussian distributions yield bell-shaped frequency curves having a preponderance of values around the mean with progressively fewer observations as the curve extends outward." o. Hazardous Atmosphere is defined in paragraph N25.2(g) of appendix N to part 25 as "an atmosphere that may expose maintenance personnel, passengers or flightcrew to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a confined space), injury, or acute illness." p. Inert is defined in paragraph N25.2(h) of appendix N to part 25. It states "the tank is considered inert when the bulk average oxygen concentration within each compartment of the tank is 12 percent or less from sea level up to 10,000 feet altitude, then linearly increasing from 12 percent at 10,000 feet to 14.5 percent at 40,000 feet altitude, and extrapolated linearly above that altitude." q. Itting is is defined in paragraph N25.2(i) as "a process where a noncombustible gas is introduced into the ullage of a fuel tank so that the ullage becomes non-flammable." r. Lean Fuel Vapor/Air Mixture is a fuel vapor/air mixture that contains a concentration of fuel molecules below that which will support combustion. 7 EFTA00796630 9/19/08 AC 25.98I-2A s. Monte Carlo Analysis is defined in paragraph N25.2(j) of appendix N to part 25 as "the analytical method that is specified in this appendix (appendix N to part 25) as the compliance means for assessing the fleet average flammability exposure time for a fuel tank." Paragraph N25.3(a) requires analysis be performed in accordance with the methods and procedures defined in the User's Manual referenced in paragraph 4e(11) of this AC. t. Oxygen evolution is defined in paragraph N25.2(k) of appendix N to part 25. It "occurs when oxygen dissolved in the fuel is released into the ullage as the pressure and temperature in the fuel tank are reduced." u. Rich Fuel Vapor/Air Mixture is a fuel vapor/air mixture that contains a concentration of fuel molecules above that which will support combustion. v. Warm Day Case is that portion of the Monte Carlo Analysis during ground or takeoff/climb phases of flights that begin with a sea level ground ambient temperature of 80 degrees F (standard day plus 21 degrees F) or above, from the flammability analysis done for overall ground operations or warm day takeoff/climb phases. w. Standard Deviation is defined in paragraph N25.2(I) of appendix N to part 25 as "a statistical measure of the dispersion or variation in a distribution, equal to the square root of the arithmetic mean of the squares of the deviations from the arithmetic means." x. Transport Effects is defined in paragraph N25.2(m) of appendix N to part 25 as "the change in fuel vapor concentration in a fuel tank caused by low fuel conditions and fuel condensation and vaporization." The change caused by low fuel conditions is also referred to as "mass loading" (see paragraph 4(e)(4)). y. Ullaee is defined in paragraph N25.2(n) of appendix N to part 25 as "the volume within the fuel tank not occupied by liquid fuel." z. Equivalent Conventional Unheated Aluminum Wine Tank is defined in § 25.981(b)(3)(i) as "an integral tank in a unheated semi-monocoque aluminum wing of a subsonic airplane that is equivalent in aerodynamic performance, structural capability, fuel tank capacity and tank configuration to the designed wing." aa. Body Tank is a fuel tank installed entirely inside the fuselage of an airplane in a compartment with no tank surface exposed to outside air flow during flight, e.g., an auxiliary fuel tank installed in the cargo compartment of an airplane. See paragraph 4e(11), "Fuel Tank Flammability Assessment Method User's Manual." 6. Regulatory Background. a. Amendment 25-11 to part 25 introduced the requirements of § 25.981 about limiting temperatures in fuel tanks to prevent ignition of fuel vapors in the fuel tanks from hot surfaces. Advisory Circular 25.981-1A, which was issued in 1972 (now 8 EFTA00796631 9/19/08 AC 25.98I-2A canceled), provided guidance that included failure modes that should be considered when determining compliance with the fuel tank surface temperature requirements defined in § 25.981. b. Other sections of part 25 require prevention of ignition sources from lightning (§ 25.954) and from failures in the fuel tank system (§§ 25.901 and 25.1309). Sections 25.901 and 25.1309 set forth the provisions to evaluate the fuel tank system and show that "no single failure or malfunction or probable combination of failures will jeopardize the safe operation of the airplane...." However, service history has shown that ignition sources have developed in airplane fuel tanks because of external ignition sources, and internal ignition sources resulting from unforeseen failure modes, manufacturing and maintenance errors or factors that were not considered at the time of original certification of the airplane. c. Section 25.981, as amended by Amendment 25-102, was adopted to provide improved standards for preventing ignition sources within fuel tanks and minimizing the exposure to operation of transport category airplanes with flammable vapors in the fuel tanks. Under Amendment 25-102, the title of § 25.981 was revised to "Fuel tank ignition prevention," and paragraphs (a) and (b) were revised to address the prevention of ignition sources within the fuel tanks. Guidance on these paragraphs is provided in AC 25.981- 1B, Fuel Tank Ignition Source Prevention Guidelines (or latest revision). Amendment 25-102 also added a new paragraph (c), which requires minimization of the formation of flammable vapors in the fuel tanks, or mitigation of any hazards if ignition does occur. This provision was included in § 25.981(c), which was intended to require design practices that reduce exposure to operation with flammable vapors in transport category airplane fuel tanks to the lowest practical level. 7. Current Requirements. The Fuel Tank Flammability Reduction (FTFR) rule titled "Reduction of Fuel Tank Flammability in Transport Category Airplanes" is effective as of September 19, 2008. The 2008 FTFR rule included an amendment to part 25 fuel tank flammability requirements, part 26 (Continued Airworthiness and Safety Improvements for Transport Category Airplanes) by adding a new subpart D, Fuel Tank Flammability, and amendments to certain operational rules associated with the subpart D requirements. This AC provides guidance for § 25.981, as amended by Amendment 25-125, and the continuous airworthiness requirements of part 26, subpart D. These rules apply to new certification and to certain existing type design approval holders (DAH). The 2008 FTFR also included operational requirements related to the part 26, subpart D, requirements. Guidance for affected operators will be issued later. The following table summarizes the amendments and the applicability of each amendment. Because of the complexity of these requirements, you should refer to the specific regulations for complete details. 9 EFTA00796632 9/19/08 AC 25.98I-2A TABLE 1 Summary of Regulatory Changes that were made by the 2008 Fuel Tank Flammability Reduction rule Summary of Rules 14 CFR Description of Requirement Applies To § 25.5 Incorporation by Reference Incorporates the Fuel Tank Flammability Users's Manual by reference § 25.981, Fuel Tank Explosion Prevention Paragraph (a) provides ignition prevention requirements; (b) specifies flammability exposure limits for different fuel tank types and mandates use of fuel tank flammability assessment method; (c) provides the option of using Ignition Mitigation Means (IMM) instead of meeting the paragraph (b) flammability limits; and (d) contains requirements for airworthiness limitation items (ALI), including critical design configuration control limitations (CDCCL), for ignition prevention means, IMM or FRM. Applicants for TCs for transport category airplanes and design changes to those certificates. Appendix M, Fuel Tank System Flammability Reduction Means Establishes performance, reliability and reporting requirements for flammability reduction means (FRM). Applicants for approval of FRM. Appendix N, Fuel Tank Flammability Exposure and Reliability Analysis Defines the fuel tank flammability exposure analysis model (Monte Carlo) including definitions, input variables and data tables that must be used in the analysis. Any person required to perform flammability exposure analysis. Part 26 Continued Airworthiness and Safety Improvements for Transport Category Airplanes § 26.5 Applicability Table Provides an overview of the applicability of part 26. It provides guidance in identifying what sections apply to various types of entities. The specific applicability of each subpart and section is specified in the regulatory text. Subpart D addresses fuel tank flammability. Applicants for TCs, and changes to those TCs for transport category airplanes. Manufacturers of certain airplane models. Part 26, subpart D Fuel Tank Flammability. TCs, and design changes to those TCs for transport category airplanes. Manufacturers of certain airplane models. 10 EFTA00796633 9/19/08 AC 25.98I-2A § 26.31, Definitions Provides definitions of certain terms used in part 26, subpart D. TCs, and design changes to those TCs for transport category airplanes. Manufacturers of certain airplane models. § 26.33, Holders of Type Certificates: Fuel tank safety Require flammability exposure analysis of all fuel tanks within 150 days after September 19, 2008. If below 7 percent, no flammability reduction required. If above 7 percent, normally emptied, and any portion of tank is located in fuselage, must develop service instructions for installation of an 1MM or FRM that meets appendix M to part 25 and must submit ALI by September 20, 2010. If above 7 percent, and other tank type, must develop service instructions to incorporate an 1MM (meet § 25.981(c)) or FRM to reduce flammability exposure to 7 percent and must submit ALI by September 20, 2010. Service instructions are required by September 20, 2010. TC holders. Large transport category passenger - carrying airplanes, with passenger capacity of 30 or more or a payload of 7500 lbs. or more (original TC or later increase). § 26.35, Changes to type certificates affecting fuel tank flammability STC and field approval holders: STC and field approval holders for normally empty fuel tanks for large transport category airplanes, with passenger capacity of 30 or more or a payload of 7500 lbs. or more (original TC or later increase). Applicants for future STCs or amendments to TCs that affect fuel tank system or IMM/FRM on passenger -Require flammability exposure analysis of all normally emptied fuel tanks installed under STC or field approval by September 19, 2009. Require impact assessment of normally emptied fuel tanks installed by STC and field approval on all Airbus airplane models and certain Boeing airplane models (those with normally emptied heated center wing tanks) on 1MM or FRM developed by TC holder to determine if any ALI has been compromised by March 21, 2011. Require development of service instructions to correct designs that compromise ALI defined by TC holder by September 19, 2012. Applicants for STCs or amendments to TCs: Require flammability exposure analysis of affected fuel tanks by September 19, 2009, or before certification, whichever occurs later. For changes to existing fuel tank capacity and application made on or after September 19, 2008, must comply with § 26.33. For changes that may increase the flammability exposure of a tank for which § 26.33 requires FRM or 1MM and application made on or after September 19, II EFTA00796634 9/19/08 AC 25.98I-2A 2008, requires impact assessment of fuel tanks and other STCs, on 1MM or FRM developed by TC holder to determine if any ALI has been violated by March 21, 2011, or before certification, whichever is later. Applicants for any pending and future fuel tank that is normally empty must comply with the requirements of § 25.981, Amendment 25-125. Require development of service instructions to correct designs that compromise ALI defined by TC holder by March 19, 2012 or before certification, whichever is later. carrying airplanes. § 26.37, Pending type certification projects: Requires compliance with § 25.981, Amendment 25-125, if the application was made on or after June 6, 2001. Pending certification Fuel tank flammability projects for large transport category passenger- carrying airplanes. § 26.39, Newly produced Requires fuel tanks on affected airplanes (produced under airplanes: Fuel tank FAA production certificates) for which application is Certain Boeing flammability made for original certificates of airworthiness or for export airworthiness approval after September 20, 2010, meet the flammability requirements as stated above for airplane models, both passenger carrying and § 26.33. cargo. b. Section 25.981 together with appendices M and N to part 25 and the Fuel Tank Flammability User's Manual incorporated by reference (see § 25.5), Amendment 25-125, provide flammability limits and the method for determining the flammability of fuel tanks. The flammability limits for fuel tanks that are normally emptied and have any portion of the tank located within the fuselage contour must meet the 3 percent average and 3 percent warm day flammability exposure limits in appendix M to part 25, as required by § 25.981(b)(2). Section 25.981(b) limits the flammability exposure of all other fuel tanks to either 3 percent average, or that of a fuel tank within the wing of the airplane model being evaluated, whichever is greater. If the wing is not a conventional unheated aluminum wing, § 25.981(b) requires the analysis be based on that of an assumed Equivalent Conventional Unheated Aluminum Wing Tank. If a flammability reduction means (FRM), such as nitrogen inerting, is used, additional reliability requirements are provided in appendix M to part 25. Appendix N specifies the requirements for conducting the flammability exposure analysis required to show compliance to § 25.981 and appendix M. Appendix N provides the ability to perform a qualitative analysis for fuel tanks installed in aluminum wings provided it substantiates the fuel tank is a conventional unheated wing tank. Section 25.981(c) retains the option 12 EFTA00796635 9/19/08 AC 25.98I-2A of using ignition mitigation means (IMM), for example reticulated polyurethane foam, to address fuel tank flammability requirements of § 25.981. It also extends the existing requirements for development of critical design configuration control limitations (CDCCL) for ignition prevention, that were formerly in paragraph (b), to any FRM or IMM and places the amended requirement in § 25.98I(d). c. The amendment also includes continued airworthiness and safety improvement requirements that are contained in a new subpart D to part 26. The new subpart D includes §§ 26.33, 26.35, 26.37 and 26.39. These sections specify different compliance requirements for the affected DAHs. The affected airplanes include those with a seating capacity of 30 passengers or more, or a payload of 7500 pounds or more. Appendix 1 of this AC provides a list of affected models, and Appendix 3 of this AC provides guidance on compliance with these requirements. The intent of § 25.981(b) is to require that the exposure to the formation or presence of flammable vapors is limited to specific values for fuel tanks located within the wing and fuselage contour. The flammability limits for the specific tank type are summarized in the following table: 13 EFTA00796636 9 I 9 08 AC 25.98I-2A TABLE 2 Summary of Flammability Limits for the Specific Tank Type Airplanes Affected Category of Action (Cert. Projects Include TCs, ATCs, & STCs) Applicable Regulations All part 25 transports Turbine powered large transports with Max Pass? 30 or Max Payload ≥ 7500 Lb (' See note) see appendix I for list of airplane models Future applications for new TCs § 25.981(b) Fleet Flammabi ity Exposure (Percent Exposure Time) Pending TC applied on or after June 6, 2001 § 26.37 Pending or Future STC or ATC for normally emptied tanks Future STC or ATC If changes existing fuel tank capacity § 26.35(dX2) Normally Emptied & Any Portion Inside Fuselage Appendix M (Flam S 3% Plus 3% warm day limit) All Other Fuel Tanks 3 percent or equal to conventional unheated aluminum wing tank, whichever is greater Production cut-in (After September 19, 2010) Fleet retrofit § 26.35(dX3) § 26.39 (Boeing) & Ops Rules (Airbus) § 26.33(c) & Ops Rules If Flam > 7%, Do appendix M Reduce flammability to: S 3% average and 5 3% warm day limit Flammability 5 7% * Applies to transport category, passenger carrying airplanes for which the state of manufacture issued the original certificate of airworthiness or export airworthiness approval on or after January 1, 1992. Section 26.39 production cut-in applies to both cargo and passenger airplanes. 14 EFTA00796637 9/19/08 AC 25.98I-2A 8. Compliance Demonstration. a. Showing Compliance with § 25.981(b) or (c). Section 25.981 provides two options for addressing the hazards associated with fuel tank flammability: • Controlling fuel tank flammability to specified levels, and • Mitigating the hazards if ignition of the fuel vapors occurs. (1) The first means, as provided in § 25.981(b), shows that the flammability of a fuel tank does not exceed the limits defined in the regulation. When this method of compliance is used, a flammability analysis is required to establish the flammability of the fuel tank, and incorporation of an FRM in any fuel tank to reduce the flammability of any tank that exceeds the applicable flammability limit. Guidance for determining the flammability of a fuel tank is provided in paragraph 10 of this AC. Guidance for incorporating an FRM, including pressurization of a fuel tank, temperature control of a fuel tank, limiting fuel properties and fuel tank inerting, are provided in paragraph 9 of this AC. (2) Compliance with § 25.981(b) is not required if the hazards of ignition of fuel vapors are mitigated by use of an Ignition Mitigation Means (IMM) meeting the requirements of § 25.981(c). Guidance for demonstrating compliance using IMM is provided in paragraph 12 of this AC. Examples of IMM include filling the tank with polyurethane foam, metallic foils, demonstrating the structure can withstand an explosion, or explosion suppression systems. Since IMM mitigates the effects of ignition so that it is not hazardous, there is no requirement to determine the fuel tank flammability, if this method is used to demonstrate compliance. b. Showing Compliance with § 25.981(d). Appendix 2 of this AC includes guidance for establishing CDCCL relating to FRM or IMM for the fuel tank system. c. Showing Compliance with §§ 26.33 and 26.35. Specific guidance for compliance with the continued operational safety requirements contained in part 26, subpart D is provided in appendix 3 of this AC. 15 EFTA00796638 9/19/08 AC 25.98I-2A 9. General Considerations — Fuel Tank Flammability. a. Formation of Flammable Vapors. The critical considerations in controlling exposure to operation with flammable mixtures in the tank include the control of formation of flammable vapors and/or oxygen concentration. Factors influencing the formation of flammable vapors include fuel type and properties, fuel temperature, pressure in the tank, and any design feature that significantly increases the potential for fuel mists to be created. The time a fuel tank is flammable determined by the Monte Carlo analysis is based upon the assumption that design features needed to prevent spraying and misting of fuel in the tank have been incorporated into the design so these factors are not considered. Rather, the fuel properties and temperature and pressure in the fuel tank are used to determine when the fuel tank is flammable. General design practices that affect the overall flammability risk are described below. Airplane designs submitted for FAA evaluation will be evaluated against these practices. b. Design Practices to Minimize Flammability Exposure. (1) Misting and sloshing. The flammability of fuel vapors in a fuel tank can be dramatically influenced by agitation, sloshing, spraying, or misting of fuel. These processes increase the surface area of the fuel allowing more fuel vapors to evolve from the fuel, which results in a higher concentration of fuel molecules in the ullage space. Design practices that reduce the potential for fuel agitation, sloshing, spraying and misting should be incorporated into the design so that flammability is minimized. Examples of proven design practices include installation of sufficient baffling in the tanks to reduce sloshing, and returning any fuel used to cool fuel pumps to the bottom of the tank. Section 6 of SAE Document AIR 1662 describes recommended design practices for minimizing hazards associated with electrostatic charging in fuel tanks. Several of these practices relate to minimizing the formation of flammable vapors, including: (a) Introducing fuel at low velocity near the bottom of fuel tanks so that the inlet is covered early in the refueling or fuel transfer process. (b) Directing the fuel flow onto a grounded conducting surface to reduce electrostatic charge build-up. (c) Using a balanced distribution system to make sure that all fuel tank bays are filled to equal levels to assist in reducing fuel velocity (this minimizes charge relaxation time and mist formation). These practices greatly reduce the presence of fuel mist that will broaden the flammability range of the fuel at the lean end and cause flammable vapors at temperatures well below the flash point. Appendix N to part 25, paragraph N25.4(3)(ii), defines the flammability envelope that must be used for the flammability exposure analysis. The flammability envelope is a function of the flash point of the fuel selected by the Monte Carlo flammability assessment methodology defined in paragraph 4e(11) of 16 EFTA00796639 9/19/08 AC 25.98I-2A this AC. This determination if the fuel tank is flammable is based upon the assumption that design precautions described in this paragraph have been implemented. (2) Fuel temperature is one of the key factors that determine fuel tank flammability in unpressurized fuel tanks approved for use with common Jet A type fuels. The most effective methods for controlling fuel tank temperature may differ between different fuel tanks, according to their exposure to the risk. For instance, fuel tanks located in conventional unheated aluminum wings of subsonic transport category airplanes, with little or no heat input from airplane systems or from other adjacent fuel tanks that have large surface areas that allow cooling of the fuel, have been analyzed and shown to meet the intent of the regulation. Fuel tanks located within the fuselage contours or other tanks located within the wing that do not cool require more design attention. For example, auxiliary fuel tanks located in the cargo compartment or pressurized areas, tanks located in the center wing box, horizontal stabilizer tanks, tanks with small surface areas exposed to airflow, and tanks made from materials that act as insulators, may have less ability to reject heat to ambient air, both on the ground and in flight, and may be subject to heat sources from equipment located nearby in the fuselage such as the air conditioning packs that supply cool air to the cabin. For fuel tanks that, because of installation location and/or other factors, do not meet the applicable flammability limits, an FRM or IMM is needed to comply. (3) Fuel types. The proposal for fuels for an airplane type is submitted to the FAA by the applicant for approval, and once approved the fuels are shown in the AFM. The definitions of LFL and UFL define the fuel temperature at which a fuel tank can be expected to be flammable. From the definitions, it can easily be seen that fuel flash point is key. Currently, Jet-A and Jet-A 1 are the predominant fuels used in commercial aviation. Because of this, wing tanks are commonly not flammable as the fuel temperature is typically below the LFL. However, the heat input to any tank can push tanks fueled with Jet-A/A1 into the flammable range. A fuel such as JP-4 has the reverse effect. The flash point is below normal ambient temperatures during ground operations, resulting in more flammability exposure for typical wing tanks and less for tanks that are insulated from outside air, such as CWT. The higher temperature tanks are less flammable because the fuel temperature is above the UFL more of the time, resulting in an over-rich condition. (a) Appendix N to part 25 defines a typical transport category airplane fuel based upon a survey of fuels drawn from operating airplanes as shown in paragraph 4e(10) of this AC. The fuels include Jet A/A1 flash point distribution and also lower flash point fuels commonly used in China and Russia. This distribution is also included in appendix N to part 25 and the Monte Carlo analysis in the paragraph 4d(11) of this AC. For consistency across applicants, and for simplicity, each applicant is required to apply the distribution defined in appendix N unless the FAA finds that this distribution is not representative of the fuels that could be expected to be used on the particular airplane being evaluated. 17 EFTA00796640 9/19/08 AC 25.98I-2A (b) If the use of low flash point fuels, such as JP-4, is proposed as an approved fuel, the fuel properties defined in the User's Manual may not provide a representative flammability exposure analysis. Use of JP-4 type fuels on a typical transport category airplane may significantly increase operational exposure to flammable vapors. Therefore, modification of the flammability analysis and incorporation of additional flammability reduction capability, such as improved FRM performance, may be required to mitigate the increased exposure created by continuous use of such fuels. (4) Fuel tank ullage sweeping is the introduction of air into a fuel tank and dumping the fuel vapor overboard to reduce the concentration of fuel vapors in the ullage. This means would result in significant emission of fuel vapors into the atmosphere and would likely not meet emissions standards, unless these vapors were removed prior to dumping the air into the atmosphere. Ullage sweeping would not likely have a significant effect on the bulk average fuel temperature of the affected fuel tank and it would not decrease the oxygen content of the ullage. Therefore, it would not provide a significant reduction in the flammability exposure as determined by the flammability assessment method required by part 25, appendix N. (5) Controlling oxygen concentration. The accepted level for tank inerting used by the military is to reduce the oxygen concentration in the tank ullage to less than 9 percent. This is the standard established in the 1950s for a zero flammability risk design because ignition sources (hostile munitions) are a likely event in military missions. The standard precludes "cool flame" ignition, where the pressure rise is relatively low. The actual oxygen concentration needed to prevent a catastrophic fuel tank rupture during an ignition event per FAA testing (reference 4(e)(8)), and military live fire testing using incendiary rounds (reference 4 (e)(7)), is higher. The higher the oxygen concentration, the higher the pressure that is developed in the fuel tank during a combustion event. Therefore, the applicant may establish the acceptable oxygen concentration based upon evaluation of the structural capability and maximum peak pressure of the fuel tank. The oxygen concentration defined in the definitions paragraph of this AC is an acceptable benchmark for transport category airplane fuel tanks inerted with nitrogen, without additional substantiation. The oxygen concentration limit was established using nitrogen enriched air to displace oxygen. The allowable oxygen concentration varies with the type of inerting gas used. If another inerting gas such as carbon dioxide is used the applicant must substantiate the allowable oxygen concentration needed to show compliance. c. STC and amended TC applicants for design changes to install a Normally Emptied fuel tank must comply with § 25.981 at Amendment 25-125. The guidance in this AC primarily addresses means of compliance with the new tank itself. However, these applicants must also comply with all applicable CDCCLs established by the TC holder. 18 EFTA00796641 9/19/08 AC 25.98I-2A 10. Determining Fuel Tank Flammability. a. Acceptable Means of Determining the Flammability Exposure. There are two means of establishing the flammability of a fuel tank. The method that is acceptable depends upon the flammability level that the tank is required to meet. Paragraph N25.1(a) of appendix N to part 25 allows for using a qualitative method if it substantiates the fuel tank is a conventional unheated aluminum wing tank. The criteria listed below describe the characteristics of conventional unheated aluminum wing tanks. For all other fuel tanks, the Monte Carlo Model defined in paragraph 4e(11) of this AC must be used as required by appendix N. (I) Qualitative flammability assessment. (a) A conventional unheated aluminum wing tank is a conventional aluminum structure, integral tank of a subsonic transport airplane wing, with minimal heating from airplane systems or other fuel tanks and cooled by ambient airflow during flight. Heat sources that have the potential for significantly increasing the flammability exposure of a fuel tank would preclude the tank from being considered "unheated." Examples of such heat sources that may have this effect are heat exchangers, adjacent heated fuel tanks, transfer of fuel from a wanner tank, and adjacent air conditioning equipment. Thermal anti-ice systems and thermal anti-ice blankets typically do not significantly increase flammability of fuel tanks. For these tanks, a qualitative assessment showing equivalency to the unheated aluminum wing fuel tank may be acceptable when considered with the following: 1 A description of the airplane configuration, (including subsonic, wing construction, etc.), 2 A listing of any heat sources in or adjacent to the fuel tank, 3 The type of fuel approved for the airplane, 4 The tank operating pressure relative to ambient static pressure, 5 The tank is uninsulated and made of aluminum, and 6 The tank has a large aerodynamic surface area exposed to outside air to transfer heat from the tank. (b) Fuel tanks with an aerodynamic surface area to volume ratio (surface area/volume) greater than 1.0 have been shown to meet these criteria. Fuel tanks with a ratio less than I are not considered conventional unheated aluminum wing tanks. The aerodynamic surface area includes the area of the integral aluminum wing fuel tank that is exposed to outside air. It does not include any portion of a fuel tank that is shielded from free stream airflow, such as the front and rear spar, or an area under a fairing or wing thermal blanket. 19 EFTA00796642 9/19/08 AC 25.98I-2A (c) Wing tanks that do not meet the criteria above for use of the qualitative method must use the Monte Carlo analysis (ref. appendix N). For example, if a fuel tank were made of composites, the applicant would need to show compliance using one of the two alternatives provided in § 25.981(b) for wing tanks. One alternative would require the applicant to conduct an assessment of the fleet average flammability exposure for an equivalent conventional unheated aluminum wing tank meeting the criteria above, on the airplane type for which approval is sought. This would establish the maximum allowable flammability level for the actual composite wing fuel tanks. The second alternative provided in § 25.981(