CS-25 N25 Fuel Tank Flammability Exposure

(See AMC to Appendix N) N25.1 General (a) This appendix specifies the requirements for conducting fuel tank fleet average flammability exposure analyses required to meet CS 25.981(b) and Appendix M. This appendix defines parameters affecting fuel tank flammability that must be used in performing the analysis. These include parameters that affect all aeroplanes within the fleet, such as a statistical distribution of ambient temperature, fuel flash point, flight lengths, and aeroplane descent rate. Demonstration of compliance also requires application of factors specific to the aeroplane model being evaluated. Factors that need to be included are maximum range, cruise mach number, typical altitude where the aeroplane begins initial cruise phase of flight, fuel temperature during both ground and flight times, and the performance of an FRM if installed (See AMC to appendix N, N25.1(a)). (b) For fuel tanks installed in aluminium wings, a qualitative assessment is sufficient if it substantiates that the tank is a conventional unheated aluminium wing tank (See AMC to Appendix N25.1(b)). N25.2 Definitions (a) Bulk Average Fuel Temperature means the average fuel temperature within the fuel tank or different sections of the tank if the tank is subdivided by baffles or compartments. (b) Flammability Exposure Evaluation Time (FEET). The time from the start of preparing the aeroplane 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. (c) Flammable. With respect to a fluid or gas, flammable means susceptible to igniting readily or to exploding (ref. CS-Definitions). A non-flammable ullage is one where the fuel-air vapour is too lean or too rich to burn or is inert as defined below. For the purposes of this appendix, 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. (d) Flash Point. The flash point of a flammable fluid means the lowest temperature at which the application of a flame to a heated sample causes the vapour to ignite momentarily, or “flash.” Table 1 of this appendix provides the flash point for the standard fuel to be used in the analysis. (e) Fleet average flammability exposure is the percentage of the flammability exposure evaluation time (FEET) the fuel tank ullage is flammable for a fleet of an aeroplane type operating over the range of flight lengths in a world-wide range of environmental conditions and fuel properties as defined in this appendix. (f) Gaussian Distribution is 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. (g) Hazardous atmosphere. An atmosphere that may expose maintenance personnel, passengers or flight crew to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a confined space), injury, or acute illness. (h) Inert. For the purpose of this appendix, 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. (i) Inerting. A process where a non-combustible gas is introduced into the ullage of a fuel tank so that the ullage becomes non-flammable. (j) Monte Carlo Analysis. The analytical method that is specified in this appendix as the compliance means for assessing the fleet average flammability exposure time for a fuel tank. (k) Oxygen evolution occurs when oxygen dissolved in the fuel is released into the ullage as the pressure and temperature in the fuel tank are reduced. (l) Standard deviation is 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. (m) Transport Effects. For purposes of this appendix, transport effects are the change in fuel vapour concentration in a fuel tank caused by low fuel conditions and fuel condensation and vaporization. (n) Ullage. The volume within the fuel tank not occupied by liquid fuel. N25.3 Fuel tank flammability exposure analysis (a) A flammability exposure analysis must be conducted for the fuel tank under evaluation to determine fleet average flammability exposure for the aeroplane and fuel types under evaluation. For fuel tanks that are subdivided by baffles or compartments, an analysis must be performed either for each section of the tank, or for the section of the tank having the highest flammability exposure. Consideration of transport effects is not allowed in the analysis. (b) The following parameters are defined in the Monte Carlo analysis and provided in paragraph N25.4: (1) Cruise Ambient Temperature – as defined in this appendix. (2) Ground Temperature – as defined in this appendix. (3) Fuel Flash Point – as defined in this appendix. (4) Flight length Distribution –that must be used is defined in Table 2 of this appendix. (c) Parameters that are specific to the particular aeroplane model under evaluation that must be provided as inputs to the Monte Carlo analysis are: (1) Aeroplane Cruise Altitude (2) Fuel Tank quantities. If fuel quantity affects fuel tank flammability, inputs to the Monte Carlo analysis must be provided that represent 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 EASA approved fuel management procedures. (3) Aeroplane cruise Mach Number. (4) Aeroplane maximum Range (5) Fuel Tank Thermal Characteristics. If fuel temperature affects fuel tank flammability, inputs to the Monte Carlo analysis must be provided that represent the actual bulk average fuel temperature within 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 must be provided either for each section of the tank or for the section of the tank having the highest flammability exposure. Input values for this data must 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. (6) Maximum aeroplane operating temperature limit as defined by any limitations in the Aeroplane Flight Manual. (7) Aeroplane Utilization. The applicant must provide data supporting the number of flights per day and the number of hours per flight for the specific aeroplane model under evaluation. If there is no existing aeroplane fleet data to support the aeroplane being evaluated, the applicant must provide substantiation that the number of flights per day and the number of hours per flight for that aeroplane model is consistent with the existing fleet data they propose to use. (8) Aeroplane climb & descent profiles in accordance with the aircraft performance data documented in the Aircraft Flight Manual. (d) Fuel Tank FRM Model. If FRM is used, an Agency approved Monte Carlo program must be used to show compliance with the flammability requirements of CS 25.981 and Appendix M of this part. 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 flammability exposure evaluation time and under the full range of expected operating conditions, when the FRM is operating properly but fails to maintain a non-flammable fuel tank because of the effects of the fuel tank vent system or other causes, (2) If dispatch with the system inoperative under the Master Minimum Equipment List (MMEL) is requested, the time period assumed in the reliability analysis shall be consistent with the proposed rectification interval, depending on aeroplane utilisation, (3) Frequency and duration of time periods of FRM inoperability, substantiated by test or analysis, caused by latent or known failures, including aeroplane system shut-downs and failures that could cause the FRM to shut down or become inoperative, (4) Effects of failures of the FRM that could increase the flammability exposure of the fuel tank, (5) Oxygen Evolution: If an FRM is used that is affected by oxygen concentrations in the fuel tank, the time periods when oxygen evolution from the fuel results in the fuel tank or compartment exceeding the inert level. The applicant must include any times when oxygen evolution from the fuel in the tank or compartment under evaluation would result in a flammable fuel tank. The oxygen evolution rate that must be used is defined in the FAA document “Fuel Tank Flammability Assessment Method User's Manual”, dated May 2008 (or latest revision), document number DOT/FAA/AR–05/8. (6) If an inerting system FRM is used, the effects of any air that may enter the fuel tank following the last flight of the day due to changes in ambient temperature, as defined in Table 4, during a 12-hour overnight period. N25.4 Variables and data tables The following data must be used when conducting a flammability exposure analysis to determine the fleet average flammability exposure. Variables used to calculate fleet flammability exposure must include atmospheric ambient temperatures, flight length, flammability exposure evaluation time, fuel flash point, thermal characteristics of the fuel tank, overnight temperature drop, and oxygen evolution from the fuel into the ullage. (a) Atmospheric Ambient Temperatures and Fuel Properties. (1) In order to predict flammability exposure during a given flight, the variation of ground ambient temperatures, cruise ambient temperatures, and a method to compute the transition from ground to cruise and back again must be used. The variation of the ground and cruise ambient temperatures and the flash point of the fuel is defined by a Gaussian curve, given by the 50 percent value and a ± 1-standard deviation value. (2) Ambient Temperature: Under the program, the ground and cruise ambient temperatures are linked by a set of assumptions on the atmosphere. The temperature varies with altitude following the International Standard Atmosphere (ISA) rate of change from the ground ambient temperature until the cruise temperature for the flight is reached. Above this altitude, the ambient temperature is fixed at the cruise ambient temperature. This results in a variation in the upper atmospheric temperature. For cold days, an inversion is applied up to 10,000 feet, and then the ISA rate of change is used. (3) Fuel properties: (i) For Jet A and Jet A-1 fuel, the variation of flash point of the fuel is defined by a Gaussian curve, given by the 50 percent value and a ± 1-standard deviation, as shown in Table 1. (ii) The flammability envelope of the fuel that must be used for the flammability exposure analysis is a function of the flash point of the fuel selected by the Monte Carlo for a given flight. The flammability envelope for the fuel is defined by the upper flammability limit (UFL) and lower flammability limit (LFL) as follows: (A) LFL at sea level = flash point temperature of the fuel at sea level minus 5.5°C (10°F). LFL decreases from sea level value with increasing altitude at a rate of 0.55 °C (1oF) per 808 feet. (B) UFL at sea level = flash point temperature of the fuel at sea level plus 19.5°C (63.5oF). UFL decreases from the sea level value with increasing altitude at a rate of 0.55°C (1oF) per 512 feet. (4) For each flight analyzed, a separate random number must be generated for each of the three parameters (ground ambient temperature, cruise ambient temperature, and fuel flash point) using the Gaussian distribution defined in Table 1. Table 1. Gaussian Distribution for Ground Ambient Temperature, Cruise Ambient Temperature, and Fuel Flash Point Parameter Ground Ambient Temperature. Mean Temp 15.53/59.95 Neg 1 std dev 11.18/ 20.14 Pos 1 std dev 9.6/ 17.28 Temperature in Deg C/Deg F Cruise ambient Fuel Flash Point (FP) Temperature. _ -56.67/ -70 48.89/ 120 4.4/ 8 4.4/ 8 4.4/ 8 4.4/8 (b) The Flight Length Distribution defined in Table 2 must be used in the Monte Carlo analysis. Table 2. Flight Length Distribution Aeroplane Maximum Range – Nautical Miles (NM) 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Flight Length Distribution of flight lengths (Percentage of total) (NM) From To 0 200 11.7 7.5 6.2 5.5 200 400 27.3 19.9 17.0 15.2 400 600 46.3 40.0 35.7 32.6 600 800 10.3 11.6 11.0 10.2 800 1000 4.4 8.5 8.6 8.2 1000 1200 0.0 4.8 5.3 5.3 1200 1400 0.0 3.6 4.4 4.5 1400 1600 0.0 2.2 3.3 3.5 1600 1800 0.0 1.2 2.3 2.6 1800 2000 0.0 0.7 2.2 2.6 2000 2200 0.0 0.0 1.6 2.1 2200 2400 0.0 0.0 1.1 1.6 2400 2600 0.0 0.0 0.7 1.2 2600 2800 0.0 0.0 0.4 0.9 2800 3000 0.0 0.0 0.2 0.6 3000 3200 0.0 0.0 0.0 0.6 3200 3400 0.0 0.0 0.0 0.7 3400 3600 0.0 0.0 0.0 0.7 3600 3800 0.0 0.0 0.0 0.9 4.7 4.0 3.4 3.0 2.6 2.3 13.2 11.4 9.7 8.5 7.5 6.7 28.5 24.9 21.2 18.7 16.4 14.8 9.1 8.0 6.9 6.1 5.4 4.8 7.4 6.6 5.7 5.0 4.5 4.0 4.8 4.3 3.8 3.3 3.0 2.7 4.2 3.8 3.3 3.0 2.7 2.4 3.3 3.1 2.7 2.4 2.2 2.0 2.5 2.4 2.1 1.9 1.7 1.6 2.6 2.5 2.2 2.0 1.8 1.7 2.2 2.1 1.9 1.7 1.6 1.4 1.7 1.7 1.6 1.4 1.3 1.2 1.4 1.4 1.3 1.2 1.1 1.0 1.0 1.1 1.0 0.9 0.9 0.8 0.7 0.8 0.7 0.7 0.6 0.6 0.8 0.8 0.8 0.8 0.7 0.7 1.1 1.2 1.2 1.1 1.1 1.0 1.3 1.6 1.6 1.5 1.5 1.4 2.2 2.7 2.8 2.7 2.6 2.5 Aeroplane Maximum Range – Nautical Miles (NM) 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Flight Length Distribution of flight lengths (Percentage of total) (NM) 3800 4000 0.0 0.0 0.0 0.5 4000 4200 0.0 0.0 0.0 0.0 4200 4400 0.0 0.0 0.0 0.0 4400 4600 0.0 0.0 0.0 0.0 4600 4800 0.0 0.0 0.0 0.0 4800 5000 0.0 0.0 0.0 0.0 5000 5200 0.0 0.0 0.0 0.0 5200 5400 0.0 0.0 0.0 0.0 5400 5600 0.0 0.0 0.0 0.0 5600 5800 0.0 0.0 0.0 0.0 5800 6000 0.0 0.0 0.0 0.0 6000 6200 0.0 0.0 0.0 0.0 6200 6400 0.0 0.0 0.0 0.0 6400 6600 0.0 0.0 0.0 0.0 6600 6800 0.0 0.0 0.0 0.0 6800 7000 0.0 0.0 0.0 0.0 7000 7200 0.0 0.0 0.0 0.0 7200 7400 0.0 0.0 0.0 0.0 7400 7600 0.0 0.0 0.0 0.0 7600 7800 0.0 0.0 0.0 0.0 7800 8000 0.0 0.0 0.0 0.0 8000 8200 0.0 0.0 0.0 0.0 8200 8400 0.0 0.0 0.0 0.0 8400 8600 0.0 0.0 0.0 0.0 8600 8800 0.0 0.0 0.0 0.0 8800 9000 0.0 0.0 0.0 0.0 9000 9200 0.0 0.0 0.0 0.0 9200 9400 0.0 0.0 0.0 0.0 9400 9600 0.0 0.0 0.0 0.0 9600 9800 0.0 0.0 0.0 0.0 9800 10000 0.0 0.0 0.0 0.0 2.0 2.6 2.8 2.8 2.7 2.6 2.1 3.0 3.2 3.3 3.2 3.1 1.4 2.2 2.5 2.6 2.6 2.5 1.0 2.0 2.3 2.5 2.5 2.4 0.6 1.5 1.8 2.0 2.0 2.0 0.2 1.0 1.4 1.5 1.6 1.5 0.0 0.8 1.1 1.3 1.3 1.3 0.0 0.8 1.2 1.5 1.6 1.6 0.0 0.9 1.7 2.1 2.2 2.3 0.0 0.6 1.6 2.2 2.4 2.5 0.0 0.2 1.8 2.4 2.8 2.9 0.0 0.0 1.7 2.6 3.1 3.3 0.0 0.0 1.4 2.4 2.9 3.1 0.0 0.0 0.9 1.8 2.2 2.5 0.0 0.0 0.5 1.2 1.6 1.9 0.0 0.0 0.2 0.8 1.1 1.3 0.0 0.0 0.0 0.4 0.7 0.8 0.0 0.0 0.0 0.3 0.5 0.7 0.0 0.0 0.0 0.2 0.5 0.6 0.0 0.0 0.0 0.1 0.5 0.7 0.0 0.0 0.0 0.1 0.6 0.8 0.0 0.0 0.0 0.0 0.5 0.8 0.0 0.0 0.0 0.0 0.5 1.0 0.0 0.0 0.0 0.0 0.6 1.3 0.0 0.0 0.0 0.0 0.4 1.1 0.0 0.0 0.0 0.0 0.2 0.8 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 (c) Overnight Temperature Drop. For aeroplanes on which FRM is installed, the overnight temperature drop for this appendix is defined using: (1) A temperature at the beginning of the overnight period that equals the landing temperature of the previous flight that is a random value based on a Gaussian distribution; and (2) An overnight temperature drop that is a random value based on a Gaussian distribution. (3) For any flight that will end with an overnight ground period (one flight per day out of an average of number of flights per day, depending on utilization of the particular aeroplane model being evaluated), the landing outside air temperature (OAT) is to be chosen as a random value from the following Gaussian curve: Table 3. Landing Outside Air Temperature Parameter Landing Outside Air Temperature °C/ °F Mean Temperature 14.82/ 58.68 negative 1 std dev 11.41/ 20.55 positive 1 std dev 7.34/ 13.21 (4) The outside ambient air temperature (OAT) overnight temperature drop is to be chosen as a random value from the following Gaussian curve: Table 4. Outside Air Temperature (OAT) Drop Parameter OAT Drop Temperature °C/ °F Mean Temp -11.11/ 12.0 1 std dev 3.3/ 6.0 (d) Number of Simulated Flights Required in Analysis. In order for the Monte Carlo analysis to be valid for showing compliance with the fleet average and warm day flammability exposure requirements, the applicant must run the analysis for a minimum number of flights to ensure that the fleet average and warm day flammability exposure for the fuel tank under evaluation meets the applicable flammability limits defined in Table 5. Table 5. Flammability Exposure Limit Minimum Number of Maximum Acceptable Monte Carlo Maximum Acceptable Monte Flights in Monte Carlo Average Fuel Tank Flammability Carlo Average Fuel Tank Analysis Exposure (%) to meet 3% Flammability Exposure (%) to requirements 10,000 2.91 100,000 2.98 1,000,000 3.00 meet 7% requirements 6.79 6.96 7.00