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.
b. Fuel Tank FRM Model. If FRM is used, the rule requires an FAA-approved Monte Carlo Model be used to show compliance with the flammability requirements of § 25.981 and appendix M of part 25. The program must determine the time periods during each flight phase when the fuel tank or compartment with the FRM would be flammable. The following factors must be considered in establishing these time periods:
(1) Any time periods throughout the FEET and under the full range of expected operating conditions, when the FRM is operating properly but fails to maintain a nonflammable fuel tank because of the effects of the fuel tank vent system or other causes.
(2) The effect of any inerting system may be included in this analysis. The applicant should present a description of the system including its control logic, and data to substantiate its performance at lowering the ullage oxygen content to below the inert level defined in appendix N to this part to comply with the applicable average flammability level. Where the system does not provide continuous protection such as a ground based inerting system as described in paragraph 4e(5) of this AC, the degradation of the ullage inerting level with time, altitude, oxygen evolution from the fuel, air inhalation with fuel use or altitude reduction, and possible effects from vent system operation must be addressed.
c. Compliance Report. The results of the analysis described above, together with substantiation data for the assumptions used in the analysis, including test data validating the thermal analysis used, would be included in the compliance report presented to the FAA. If the tank in question requires a demonstration of equivalence, an analysis of a real or hypothetical unheated wing tank on the same airplane must also be presented.
d. Documentation and Validation of Monte Carlo Model. As discussed earlier in paragraph 10, modifications to the Monte Carlo model are limited to the additional features needed to model an FRM, tank thermal characteristics, and fuel management.
All modifications to the model’s code must be thoroughly documented and validated both through detailed analysis and flight test. The compliance report should include a summary section that lists all modifications.
11. Flammability Reduction Means. The available methods of limiting exposure to operation with a flammable ullage space in the tank include preventing the formation of flammable vapors and/or controlling the oxygen concentration. Factors that directly influence the formation of flammable vapors include the fuel type properties, fuel temperature, pressure within the fuel tank, and any design feature that increases the potential for fuel mists to be created. Design precautions described earlier within this AC to limit misting or sloshing of fuel within a fuel tank should be taken.
Demonstrating compliance when an FRM is used requires modification to the Monte Carlo Model to include a simulation of the FRM performance. The simulation must determine the times during the flight when the FRM is effective at maintaining a non- 25
flammable fuel tank. Factors that need to be considered when developing the simulation are discussed within this section.
a. Managing Heat Transfer to the Fuel Tank. In general, heat sources should not be located in or near fuel tanks, and heating from other sources, such as hydraulic heat exchangers or rejection of heat from engine systems, should be avoided unless features are incorporated to maintain fuel tank flammability equivalent to an unheated wing fuel tank. Locating heat-producing systems away from the tanks should be considered. If this is not a practical solution, installation of an FRM using thermal control may be another option. Possible technical solutions include the use of thermal insulation blankets, and/or providing ventilation and/or dedicated cooling to remove excess heat from the fuel tank or areas adjacent to the tank.
b. Cooling/Ventilation of Fuel Tanks. If the fuel tank is located in an area of the airplane where little or no cooling occurs, such as the center wing box, certain horizontal stabilizers, or auxiliary fuel tanks located in the cargo compartment, ventilation or dedicated cooling may also be an effective means of demonstrating compliance. The cooling/ventilation means should be effective under all operating conditions, including ground and flight operation, considered necessary to achieve the desired goal of showing the tank flammability is equivalent to the unheated wing tanks. Adequate cooling/ventilation may be provided for certain airplane types by means such as installation of an air gap in spaces adjacent to fuel tanks and utilizing cold air source during ground operation to cool the tank, and the use of ram air inlets for in-flight operation to transfer heat from the tank. Other means (e.g., bleeding cool air from the ECS packs into the air gap) may also be effective at providing adequate cooling/ventilation of the tank.
c. Controlling Fuel Tank Pressure. The flammability of the fuel tank is affected by the pressure in the tank. Typical transport category airplane fuel tanks are vented to the atmosphere and operate at or slightly above local ambient pressure. Fuel tanks located within the wing typically operate with a pressure equal to the local static pressure unless features are incorporated to increase the fuel tank operating pressure. Designs have been developed that use pressurization of the wing fuel tanks as a means to transfer fuel.
Pressurization of the fuel tanks also provides reduced flammability. Manufacturers have also incorporated National Advisory Committee for Aeronautics (NACA) scoops on the vent outlets of some airplane fuel tank systems to enhance engine suction feed performance. The pressure recovery factor of these scoops varies but in general is in the range of .4 to .6. Therefore fuel tank pressure during flight may be between .5 to 1.25 psi above local static pressure. The increase in tank pressure results in a reduction in fuel tank flammability.
Many recently designed auxiliary fuel tanks, located in pressurized cargo compartments, incorporate structural features that allow utilization of pneumatic pressure to transfer fuel from the fuel tank to the airplane fuel tank system. These tanks typically are designed to operate at the airplane cabin pressure which is normally equivalent to approximately 8000 feet pressure altitude. This results in a significant reduction in fuel 26
tank flammability. Controlling fuel tank pressure is one means of reducing fuel tank flammability. The Body Tank model in the Monte Carlo Model defined in paragraph 4e(11) of this AC has features that allow the user to input a pressure differential relative to ambient pressure so that fuel tank flammability under these conditions can be evaluated.
d. Fuel Tank Ullage Sweeping. A positive ventilation system has been studied to be used to “sweep” the ullage of flammable fuel vapor/air mixtures at a rate that keeps the ullage lean when the fuel temperature and pressure in the tank would otherwise define the tank as being flammable. It is discussed in the ARAC Fuel Tank Harmonization Working Group Final Report in paragraph 4e(2) of this AC. The rule requires that the fuel tank flammability be determined based upon the Monte Carlo analysis, which determines if the fuel tank ullage is flammable based on the fuel temperature and fuel tank altitude (or pressure altitude).
e. Higher Flash Point Fuels. One method of reducing fuel tank flammability is to restrict the fuel type specified in the AFM to higher flash point fuels (e.g., JP-5, 140 degrees F flash point). This method, in combination with other means, may be effective at reducing the exposure. However, as discussed in the ARAC Fuel Tank Harmonization Working Group Final Report in paragraph 4e(2) of this AC, this approach is not considered to be practical at this time. If this method were considered, the applicant would need to request FAA approval for use of a fuel type with a different flash point when conducting the flammability assessment than that required by appendix N to part 25.
f. Oxygen Concentration.
(1) Fuel Tank Inerting. Fuel tank inerting is a highly effective means of reducing or eliminating the flammability exposure within a given tank. This method eliminates flammable vapors by displacing oxygen from the ullage space of the tank with inert gas (reducing the oxygen concentration below the level that would support combustion of flammable vapors). The military has used this method to prevent fuel tank explosions of combat airplanes. While system requirements for commercial applications are different, the development of technology for separation of nitrogen gas from air has allowed the size and weight of inerting systems to be significantly reduced for commercial applications. Additional size and weight reductions may be achieved by new approaches such as catalytic conversion of fuel vapors in fuel tanks to produce carbon dioxide for use as the inert gas that is under development.
(2) Recent technology developments have made it possible to develop inerting systems that use optimized air sources that are part of the initial airplane design. These systems are capable of inerting a fuel tank during normal airplane operating conditions.
Older technology airplanes that have limited bleed air sources are less efficient and it may not be practical to provide an inerting system that inerts the tank throughout the flight envelope. On these airplanes a flammability assessment based upon the time the 27
inerting system is effective, as well as consideration of the fuel tank flammability, may be part of the overall compliance demonstration.
(3) The applicant may show that inerting is only needed for certain missions, for example during warmer days when the fuel ullage would be flammable, or parts of a particular flight to lower the fuel tank vapor/air mixture average exposure to a level that meets the flammability limit. Inerting may be achieved by supplying inert gas from onboard storage bottles, holding either gas or liquid inerting agent, on board inert gas generation systems (OBIGGS), or from a ground storage system if the tank is inerted only on the ground. Nitrogen is currently the inert gas of choice for inerting fuel tanks and the requirements in the § 25.981 address the requirements for use of nitrogen based inerting systems. Use of other gases, such as carbon dioxide, to inert a fuel tank would include many of the same considerations for markings and maintenance procedures.
However there may be other considerations that need to be addressed in the certification plan, such as gas absorbed by the fuel and the effect of the gas absorbed on the fuel system performance, such as suction feed performance.
(4) FRM designs that reduce the oxygen concentration to very low levels during cruise and increase the flow of nitrogen enriched air (NEA) into the fuel tank during decent have been found to optimize the design. One nitrogen inerting system designed for retrofit into airplanes with limited air sources has a dual flow mode that takes into consideration the performance of the air separation modules (see paragraph 4e(12) of this AC). During cruise the system operates at low flow mode where high nitrogen concentration NEA is introduced into the fuel tank to lower the oxygen concentration to very low levels. During decent the system is operated in high flow mode where the flow rate of NEA is increased. At high flow rates the nitrogen concentration of the NEA is lower.
(5) Oxygen Level. The accepted practice for tank inerting used by the military for a nitrogen based system is to reduce the oxygen concentration in the tank ullage to less than 9 percent. Data obtained from military testing that was conducted in 1991 (see paragraph 4e(7) of this AC), as well as FAA testing (see paragraph 4e(8) of this AC), shows that 12 percent oxygen concentration at sea level, will provide sufficient protection for transport category airplanes when using nitrogen based systems. As stated in the definitions paragraph of this AC, a fuel tank is considered to be inert when the bulk average oxygen concentration within each compartment of the fuel 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.
g. Oxygen Evolution. Fuel loaded on the airplane from sources vented to the atmosphere contains dissolved oxygen. Systems added to inert fuel tanks must take into account the dissolved and entrained oxygen in the fuel when loaded onto the airplane.
The applicant can either design the system to allow and compensate for the evolution of the oxygen or include provisions to remove or “scrub” the oxygen from the fuel during refueling. The effect of oxygen that may rapidly evolve from the fuel during pressure reduction conditions, such as during climb, must be accounted for in the Monte Carlo 28