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 § 25.981(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:
TABLE 2
Summary of Flammability Limits for the Specific Tank Type
Fleet Flammability Exposure (Percent Exposure Time) Airplanes Affected
Category of Action
(Cert. Projects Include TCs, ATCs, & STCs) Applicable Regulations Normally Emptied & Any Portion Inside Fuselage All Other Fuel Tanks All part 25 transports Future applications for new TCs § 25.981(b)
Pending TC applied on or after June 6, 2001 § 26.37 Pending or Future STC or ATC for normally emptied tanks § 26.35(d)(2) Appendix M
(Flam ≤ 3% Plus 3% warm day limit) 3 percent or equal to conventional unheated aluminum wing tank, whichever is greater Future STC or ATC If changes existing fuel tank capacity § 26.35(d)(3) Production cut-in (After September 19, 2010) § 26.39 (Boeing) & Ops Rules (Airbus) Turbine powered large transports with
Max Pass ≥ 30
or
Max Payload ≥ 7500 Lb (* See note) See appendix 1 for list of airplane models Fleet retrofit
§ 26.33(c) & Ops Rules
If Flam > 7%, Do appendix M
Reduce flammability to:
≤ 3% average and ≤ 3% warm day limit Flammability ≤ 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.
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.
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
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-A1 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.
(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.
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.
(1) 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 warmer 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 1 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.
(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(b) is to conduct an assessment of the actual composite wing tanks and demonstrate the fleet average flammability exposure of any tanks does not exceed three percent.
(2) Monte Carlo Analysis. When the compliance demonstration requires demonstrating a fuel tank meets a specific fuel tank flammability or the tank does not meet the qualitative assessment criteria previously discussed, both § 25.981(b)(1) and paragraph N25.3(a) of appendix N to part 25 require the flammability assessment be conducted in accordance with the methods and procedures in the FAA document, “Fuel Tank Flammability Assessment Method User’s Manual,” (User’s Manual) dated May 2008, which is incorporated by reference in § 25.5 for § 25.981 and Appendix N. This document may be obtained from the following web site:
(http://www.fire.tc.faa.gov/systems/fueltank/FTFAM.stm). Section 25.5 includes a provision permitting use of later FAA versions of the User’s Manual when FAA publishes a notice of the change in the Federal Register.
(a) This analytical method is based upon predicting the fleet average flammability exposure, and the warm day fleet average flammability exposure as applicable, to operation with flammable fuel air vapors in the fuel tank. The fuel tank flammability exposure for the tank being evaluated is calculated for the specific fleet of airplanes of interest for which approval is sought using certain standardized parameters for all airplane models and certain airplane model specific parameters. The flammability exposure calculated by the analysis is then compared to the flammability limits defined in the regulations to determine compliance. The analysis method is therefore a standardized analysis method.
(b) The fuel flash point at a given temperature and pressure is used to determine when the ullage in the tank is flammable. When the fuel temperature at a given altitude is in the range between the LFL and the UFL, the ullage is assumed to instantaneously become flammable. When using a Monte Carlo analysis, that transition is considered to occur instantaneously as pressure and temperature changes. The Monte Carlo Model is based on the assumption that any fuel being loaded during refuel is at ambient outside temperature when determining flammability exposure. Appendix N to part 25 defines the fuel properties used in the Monte Carlo Model. This includes the fuel flash point distribution and the flammability envelope of the fuel that is required to be used for the analysis.
(c) Appendix N to part 25 requires the flammability exposure be determined independently for each fuel tank. For fuel tanks that are subdivided by baffles or compartments, N25.3(a) requires an analysis be performed for each section of the tank. Alternatively, if the applicant can show that one section of the tank always has higher flammability exposure than any other section, the applicant may perform the analysis only of that one section. Within each tank where barriers or walls prevent mixing of the fuel/air mixtures, separate volumes or compartments should be treated independently to determine the worst case flammability exposure for that tank because of temperature variations within different portions of the tank. If FRM based upon inerting is used, oxygen concentration in those compartments of the tank can differ significantly from other compartments in the tank (such as an airplane model that has a center wing box that extends out into the wing, or a compartment in a fuel tank where vent inlets may create differences in temperature or oxygen concentration (if an FRM based on inerting is used)).
(d) It is possible for the critical compartment to change from one compartment of the tank to another compartment during different portions of the flight.
In this case, the flammability of the individual compartments would need to be determined throughout the flight and the times when any compartment of the tank is flammable would be included in the overall average flammability calculation.
(e) If a fuel tank is determined to exceed the applicable flammability requirements, the holder may control fuel tank flammability by incorporation of an FRM or IMM. Guidance for FRM is provided in paragraph 11 of this AC.
(3) Developing inputs to the Monte Carlo Model. The Monte Carlo Model, which randomly generates values for uncertain variables over and over, is used to simulate a process where the variables are random within defined distributions. The results of a large number of cases can then be used to approximate the results of the real world conditions. Appendix N to part 25 provides standard distributions and values that must be used when determining fuel tank flammability. Figure 1 shows the parameters that are used by the Monte Carlo Model to determine fuel tank flammability. The outlined, non-shaded boxes identify inputs that are dependent on the fuel tank design and operation and on the FRM design. The shaded boxes identify parameters that are fixed in the Monte Carlo Model.
Fuel Tank Thermal Model Flammability Reduction Means on/off Fuel Properties Airplane Climb Descent Rates Ambient Temperature Gate Time Post Landing Taxi Time Mission Length Distribution Fuel Quantity in Tank Overnight Temp Drop Gaussian Distribution Oxygen Evolution Flash Point Distribution UFL and LFL Initial Fuel Loaded Temperature 12 HR Overnight Time Average Flammability Exposure Determination
FIGURE 1
Monte Carlo Parameters to Determine Fuel Tank Flammability
(4) Monte Carlo Parameters. The fuel tank thermal model and fuel quantity in the fuel tank are user inputs needed to determine the fuel tank flammability for a fuel tank without an FRM. The performance of an FRM must also be provided by the user if an FRM is installed. User inputs to the Monte Carlo Model and standardized parameters include the following as defined in appendix N to part 25 and in the User’s Manual (defined in paragraph 4e(11) of this AC):
(a) Standardized parameters used in the Monte Carlo Model.
1 Cruise ambient temperature, N25.3(b)(1), N25.4 and Table 1 of appendix N.
2 Ground ambient temperature, N25.3(b)(2), N25.4 and Table 1 of appendix N.
3 Fuel flash point, N25.3(b)(3), N25.4 and Table 1 of appendix N.
4 Flight length distribution, in conjunction with the airplane utilization information submitted under Airplane specific inputs below.
5 Airplane climb and descent profiles (paragraph N25.3(b)(5) requires the applicant use the climb and descent profiles defined in the User’s Manual.
6 Preflight and post flight gate times; the preflight times are provided as a function of the flight time by the Monte Carlo Model in the User’s Manual and the post-flight time is a constant 30 minutes, both as defined by N25.2(b).
7 Oxygen evolution rate (included in model when FRM used, based upon User’s Manual as required by N25.3(d)(5)).
8 Overnight temperature change (when FRM is added).
(b) Airplane specific inputs: The airplane specific inputs should reflect the most severe airplane operating conditions or configuration with respect to fuel tank flammability exposure.
1 Airplane cruise altitude.
2 Fuel tank quantities. As discussed in the User’s Manual, the FAA Monte Carlo Model has a feature that allows the user to input times when the tank is full and empty so that the fuel quantity in the tank can be modeled. The model assumes a constant rate of fuel use between the specified tank full and empty times and does not account for unique fuel management techniques such as fuel transfer systems that may be incorporated for purposes such as center of gravity (C.G.) management. If fuel quantity affects fuel tank flammability, inputs to the Monte Carlo analysis may need to be modified by the user if the standard model does not provide a realistic representation of the actual fuel quantity within the fuel tank or compartment of the fuel tank throughout each of the flights being evaluated. Input values for this data must be obtained from ground and flight test data or the FAA approved fuel management procedures in the AFM.
3 Airplane cruise mach number.
4 Airplane maximum range. This parameter should be determined assuming a payload equivalent to a load factor of 75 percent for a typical two class passenger configuration for the airplane model, using year-around average passenger weights and standard average baggage and carry-on weights per AC 120-27, "Aircraft Weight and Balance Control," standard day atmosphere, zero wind, long range cruise speed and domestic or international reserves as would apply to the typical operation of the airplane.
5 Fuel tank thermal characteristics. If fuel temperature affects fuel tank flammability, the rule requires that inputs to the Monte Carlo analysis be provided that represent the actual bulk average fuel temperature within the critical compartment of the fuel tank throughout each of the flights being evaluated. For fuel tanks that are subdivided by baffles or compartments, bulk average fuel temperature inputs should be developed for each section of the tank. The temperature in any compartment that results in the highest flammability at any given time should be used in the tank flammability analysis. If one compartment in the tank always has the highest flammability exposure, the temperature in that compartment should be used in the analysis. The rule requires that input values for these data be obtained from ground and flight test data or a thermal model of the tank that has been validated by ground and flight test data. The thermal input data should reflect the most severe airplane operating conditions or configuration.
For example, for fuel tanks located near heat sources such as the airplane environmental conditioning system (ECS), also called air conditioning packs (packs), the maximum allowable number of packs should be running during test and modeling conditions.
6 Maximum airplane operating temperature limit as defined by any limitations in the AFM. This value is obtained from the flight manual for the specific airplane type.
7 Airplane Utilization. The rule requires that the applicant provide data supporting the number of flights per day and the number of hours per flight for the specific airplane model under evaluation. If there is no existing airplane fleet data to support the airplane being evaluated, the applicant must provide substantiation that the number of flights per day and the number of hours per flight for that airplane model is consistent with the existing fleet data they propose to use.
(aa) Fuel tank thermal model. A computer simulation, validated by flight test data, is one way to provide inputs for the model that simulates the thermal behavior of the tank throughout the airplane operating envelope. This model must be provided by the applicant based on the particular characteristics of the particular fuel tank that is being evaluated. The applicant may use the thermal modeling tool incorporated into the Monte Carlo Model if flight test data show that this modeling technique provides conservative prediction of fuel tank temperatures. The Monte Carlo Model includes a feature to use exponential time constants to define the heating and cooling characteristics of a fuel tank. This feature has the capability to input 6 different constants for various conditions that may be present on typical aluminum fuel tanks. While this feature is one method of modeling the fuel tank thermal characteristics so that fuel tank temperatures can be predicted, it is not an acceptable method of modeling the fuel tank if it does not provide accurate results. It may be necessary for the applicant to develop a separate fuel tank temperature model that accurately predicts fuel tank temperatures. Appendix 4 of this AC provides further guidance regarding developing a validated fuel tank thermal model for use in showing compliance for fuel tanks. This guidance will be helpful for use in developing a thermal model for a fuel tank with an FRM.
(bb) Simplified fuel tank thermal model. To simplify the certification process and avoid the need to develop a fuel tank thermal model, if an applicant so chooses, it is acceptable to determine the flammability exposure of a tank with an inerting system added by assuming the fuel temperature is otherwise always between the LFL and UFL.
(cc) Body fuel tank thermal model. If the tank is located entirely inside the fuselage of the airplane in a compartment and no tank surface is exposed to outside air flow during flight, e.g., an auxiliary fuel tank installed in the cargo compartment of an airplane, it is considered a “Body Tank.” For these fuel tanks, the Monte Carlo model defined in paragraph 4e(11) allows the calculated temperatures of the fuel tank to be controlled by the temperature of the compartment in which the fuel tanks is installed.