1. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 0 0 The Fuel Tank Flammability Assessment Method Flammability Analysis Federal Aviation Administration

2. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 1 1 The need to the limit flammability creates a need to define and assess flammability exposure assess define

3. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 2 2 What is Flammability? In order to define flammability, we first need to define: Flash point: The flash point of a flammable fluid is the lowest temperature at which the application of a flame to a heated sample causes the vapor to ignite momentarily, or “flash”, and is determined by a simple, standardized test. Lower Flammability Limit (LFL): At any point below the LFL, the fuel vapor/air mixture is too lean to burn. Upper Flammability Limit (UFL): At any point above the UFL, the fuel vapor/air mixture is too rich to burn.

4. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 3 3 Previous work has shown that the LFL and UFL can be defined, in terms of temperature as: LFL= (Flash Point - 10) - Altitude/808, UFL=(Flash Point + 63.5) - Altitude/512 (where temperature is in Deg F and altitude is in ft.) What is Flammability?

5. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 4 4 Add a slide here that shows a plot of fuel temp bounded by LFL/UFL, showing flammable range

6. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 5 5 The need to the limit flammability creates a need to define and assess flammability exposure assess define

7. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 6 6 Background The Fuel Tank Flammability Assessment Method (FTFAM) is an Excel © based macro based on work originally performed by the 1998 ARAC Fuel Tank Harmonization Working Group. It is a comparative analysis tool to examine airplane fuel tank flammability. The program utilizes Monte Carlo statistical methods to determine several unknown variables, using standardized distributions in order to calculate the fleet average flammability exposure time of a given fuel tank. From 1998 – Present, the FAA has utilized input from industry and information gained from various research activities to help refine and improve the model’s capabilities.

8. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 7 7 Basics of the Monte Carlo Method The Monte Carlo method is based on the use of random numbers and the statistics of probability to help solve problems that don’t yield to simple mathematics. Modeling of a problem is done by assigning random values, based on known distributions, to each unknown variable and calculating the results for that case. Computing the average results or range of results over a significantly large number of cases, then reduces any errors associated with the calculation.

9. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 8 8 Basics of the Monte Carlo Method (cont.) As a simple example of the Monte Carlo method, consider somebody rolling a six-sided die. What is the probability of each of the numbers coming up, if the die is rolled just one time? If the die is rolled 6 times, what will the distribution of numbers be? How about if it’s rolled 100 times, 1,000 times, or 1,000,000? In theory, the more times the die is rolled, the less error that is associated with the resulting distribution of numbers. To Try This and Other MC Examples Click on the Dice

10. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 9 9 Background (cont.) The FTFAM utilizes these techniques to generate values for several unknown variables, utilizing standardized distributions. Fuel flashpoint temperature Ambient ground temperature Ambient cruise temperature Flight mission length Additional functionality of the program: Single flight analysis (for troubleshooting) Random Number Freeze (for troubleshooting) Warm day analysis Flammability Reduction Method (FRM) effectiveness analysis

11. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 10 Program Overview – Flammability Analysis

12. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 11 Program Overview – Surrounding Environment

13. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 12 Surrounding Environment – Ambient Temperatures Ground and Cruise Ambient temperatures are calculated through Monte Carlo Method using distribution data generated by the 1998 ARAC Ground Ambient Mean Temperature = 59.95 °F Standard Deviation Below 50% = 20.14 °F Standard Deviation Above 50% = 17.28 °F Cruise Ambient Mean Temperature = -70 °F Standard Deviation = 8 °F

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15. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 14 Surrounding Environment – Ambient Temperatures With Ground and Cruise temperatures defined, we now need a method for computing the transition from ground to cruise To do so, several things must be understood and taken into account: In general, the ambient temperature decreases with increasing altitude. The rate of this decrease is referred to as the Temperature Lapse Rate. As altitude is increased, a point known as the Tropopause is reached. This is the boundary between the troposphere and stratosphere, and at this point there is no variation in ambient temperature. The tropopause ranges in height from 26,400 ft to 58,080 ft. When a cold ground ambient temperature exists, the initial temperature lapse rate can be significantly different than on warmer days. At times, it can even go positive, causing an increase in ambient temperature with altitude. This effect is referred to as a Cold Day Inversion.

16. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 15 Surrounding Environment – Ambient Temperatures The program uses the following calculations to define the standard temperature lapse rate: When the altitude is less than 10,000 ft When the altitude is greater than 10,000 ft The model defines the tropopause as the point at which the ambient temperature reaches the pre-selected cruise temperature, and at this point cuts off the lapse rate and holds the ambient temperature constant.

17. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 16 Surrounding Environment – Ambient Temperatures To deal with the issue of the cold day inversion, the program sets a different initial temperature lapse rate for flights with a ground ambient temperature less than 40 °F The lapse rate for these flights when the altitude is less than 10,000 ft is:

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19. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 18 Surrounding Environment – Ambient Temperatures For the landing portion of the flight, the program uses the same temperature lapse rates, however selects a new ground ambient temperature to ramp towards. In addition, the program takes into account that on long duration flights, an aircraft is flying into a new climate, thus causing a change in cruise temperatures. To handle this, for flights where the flight time is greater than 2 hours, the program ramps to a new cruise ambient temperature over a 45 minute period, starting just after the midpoint of the cruise cycle.

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21. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 20 Surrounding Environment – Ambient Pressure Ambient pressure is calculated through direct relationship with altitude

22. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 21 Program Overview – Mission Data

23. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 22 Mission Data – Number of Engines Number of Engines is a user input that is used by the program to calculate the climb rate of the aircraft. This climb rate also depends on the percentage of the flight time (randomly generated) to the maximum flight time of the aircraft.

24. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 23 Mission Data – Mission Length Mission Length for each flight is selected at random through the Monte Carlo method The distribution data that is used in the Monte Carlo selection process comes from data determined by the 1998 ARAC and varies depending on the maximum mission length of the aircraft. The data is generated as a percentage of total flights existing in each 200 nm block up to the maximum mission length of the aircraft.

25. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 24 Sample Mission Range Distributions Based on Maximum Range

26. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 25 Mission Data – Fuel Management The quantity of fuel in the tank at any point during flight has a direct impact on the thermal effects which determine fuel temperature, and thereby tank flammability. The model uses the quantity of fuel in the tank as part of the thermal model to determine the change in tank thermal time constants throughout the flight. To do this, the model assumes a constant rate of fuel usage between the time at which the tank starts to burn fuel and the point at which it is empty. The model uses this assumption of a linear rate of fuel usage to generate a linear change in tank thermal time constants.

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28. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 27 Mission Data – Mach Number The mach number at cruise is given by a user input Mach number changes with altitude are defined as: Altitude < 10,000 ft Mach = 0.4 10,000 ft  Altitude < 30,000 ft Altitude  30,000 ft Mach = Cruise mach number

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30. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 29 Mission Data – Miscellaneous There are a number of additional assumptions that the program makes in order to develop the full mission profile: The descent rate used is the same for all flights and is 2500 ft/min down to 4000 ft and 500 ft/min below 4000 ft. Any changes in cruise altitude are treated by the program to be instantaneous. For very short duration flights of less than 50 minutes, the climb rate is set to be 1750 ft/min. Preflight ground time is defined by the following table Postflight ground time is 30 mins for all flights.

31. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 30 Mission Data – Miscellaneous Three different cruise altitude steps are allowed to be defined by the user. These are treated differently depending on the length of each flight as follows: Flight Times < 50 min – This is a short up/down flight and the first cruise altitude step is never reached. 40% of the flight time is allocated for climb, and 60% for descent. Flight Times Between 50 and 100 minutes – The flight cruises at the first altitude step and does not step to the other levels. Flight Times Between 100 and 200 minutes – Two altitude steps are used with the step increase occurring midway through the cruise time. Flight Times > 200 min – All three altitude steps are used with the cruise time equally split between them.

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33. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 32 Program Overview – Fuel Properties

34. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 33 Fuel Properties – Flashpoint The Standard Specification for Aviation Turbine Fuels (ASTM D 1655), specifies a minimum flashpoint value of 100 °F for Jet A fuel. Similar standards for other aviation fuels also only specify a minimum value. In attempt to determine the actual flashpoint of jet fuel as used in service, the FAA conducted a study in which 293 samples were taken from both domestic and international flights. Results of this study are published in FAA report DOT/FAA/AR-07/30. The results of that study are used by the program to develop the standardized distribution of flashpoints from which a Monte Carlo analysis can be performed.

35. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 34 Mean = 120°F Std Dev. = 8°F

36. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 35 Program Overview – Fuel Tank Thermal Characteristics

37. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 36 Program Overview - Fuel Tank Thermal Characteristics As the fuel tank is heated or cooled by a change in outside air temperature or by heat input from various engines/systems, the fuel temperature will increase or decrease respectively. The program assumes that the response of the fuel temperature to these changes follows an exponential decay law. This exponential trend is represented in the program by utilizing several exponential decay time constants and equilibrium temperatures that the fuel will eventually reach if given sufficient time. In order to determine these inputs, sufficient flight test data and analysis of the thermal behavior of the fuel tank is required.

38. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 37 Program Overview - Fuel Tank Thermal Characteristics For Ground Conditions, the input data required is: Fuel temp., relative to ground ambient temp. that the fuel will reach if given sufficient time. Exponential time constant for a near empty fuel tank. Exponential time constant for a near full fuel tank. These inputs are required both for an engine on and engine off condition For Flight Conditions, the input data required is: Fuel temp., relative to TAT that the fuel will reach if given sufficient time. Exponential time constant for a near empty fuel tank. Exponential time constant for a near full fuel tank.

39. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 38 Program Overview - Fuel Tank Thermal Characteristics As discussed previously, the program assumes a linear rate of fuel usage, and therefore the time constant values move in a linear fashion from the tank full to tank empty values.

40. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 39 Program Overview - Fuel Tank Thermal Characteristics Using the tank thermal time constant and equilibrium temperature values, the program calculates the fuel temperature at each time step utilizing the following exponential decay law: This equation can also be used to solve for time constant input values, provided sufficient flight test data is obtained.

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43. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 42 Program Overview - Fuel Tank Thermal Characteristics From this example, the following inputs are determined: Equilibrium Fuel Temp. on the Ground = Ambient Temp. Equilibrium Fuel Temp. in Flight = TAT – 25°F Exponential Time Constant, Tank Near Full = 149.8 Exponential Time Constant, Tank Near Empty = 29.9 A comparison then of the model outputs and flight test data must be made to ensure accuracy of these inputs.

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46. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 45 Program Overview – Flammability Analysis

47. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 46 Answers to Questions From Tuesday 1.What are the major differences between version 9a and 10 of the FTFAM? Are there user’s manuals available from each of the previous versions? The major differences are the way in which the time constants are calculated throughout the flight, but there were numerous other changes. A file is available which lists all of the differences, and this will be made available. Either way, the only acceptable version is 10. 2.What is the basis for the time constant of 3500 for calculating oxygen evolution? Change in Partial Pressure (PP) of O2 in fuel = (PP of O2 in Ullage - PP of O2 in Fuel) *(1-e^-t/tau) Based on gradients between ullage and liquid. So, by setting tau at 3500 you are saying that under quiescent conditions, the O2 is going to come out of the fuel very very very slowly. Setting it to 100 during climb creates a much more rapid evolution of O2 out of the fuel. This is what you would expect as the fuel now is being agitated. There is a report of O2 evolution and some tests ran (DOT/FAA/AR-05/25) These time constants were derived from comparing some of these calculations to the test data, doing some curve fitting. Also, one is not required to use these time constants, or this method for taking the O2 evolution effect into account. This is merely one method that is approved. If one is able to perform tests and validate some other way of modeling it, then they certainly can. Also, the GBI report had some additional data, that report # is DOT/FAA/AR-01/63.

48. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 47 Answers to Questions From Tuesday 3.Although both the ground and cruise ambient temperatures are chosen from a standardized distribution, are the two linked? For instance, if a ground ambient is a very hot temperature, would the cruise ambient be chosen as a warmer than usual temperature? No, they are not linked...each is randomly generated independent of the other. Remember, the temperature decays according to the defined lapse rate, so depending on the temperatures generated, the cruise temperature may never be reached. 4.What is the rational behind the discretization of the mission length histograms into 200 knot blocks? No specific rationale for the 200 nm blocks...at least not that we know of. Just how the data was broken down at ARAC, and then carried over to this version

49. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 48 Program Overview The program itself is split into several separate worksheets, which can be categorized as shown below. In all of these worksheets, a yellow cell denotes a user input cell. Any cell not shaded yellow must remain unchanged by the user unless approved by the FAA

50. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 49 Program Overview – Worksheet Descriptions Intro Provides a brief statement of the model’s intended purpose as well as notes and limitations of its use. User Inputs & Results Main interface of the FTFAM. This worksheet contains all user inputs necessary for performing a Monte Carlo flammability analysis as well as results from the analysis. Single Flight Allows the user to simulate and evaluate a particular flight scenario, either by entering specific data or by selecting a flight from the MC analysis. Results from this single flight are also displayed here.

51. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 50 Program Overview – Worksheet Descriptions FRM Allows the user to evaluate the effectiveness of an FRM. It contains all necessary user inputs and the results for an FRM analysis. This worksheet is only needed if an FRM analysis is being conducted Summary of n Cases Displays the results of each flight in tabular format, sorted by the percentage of flammable time. Internal Calculations Worksheets These four worksheets contain all of the essential information processed by the model. All data inputs, claculated values, and results are stored here for use by the program and/or user. These worksheets should not be modified by the user in any way, and information contained in them is provided to the user for troubleshooting purposes only.

52. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 51 User Inputs & Results Worksheet

53. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 52 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Fuel Tank Usage Data Body Tank Input Data Tank Thermal Data Multi-Flight Monte Carlo Data

54. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 53 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Maximum range of aircraft Number of engines OAT cutoff temperature Flight Data Fuel Tank Usage Data Body Tank Input Data Tank Thermal Data Multi-Flight Monte Carlo Data

55. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 54 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Cruise Mach number Tank ram recovery Cruise altitude steps Fuel Tank Usage Data Body Tank Input Data Tank Thermal Data Multi-Flight Monte Carlo Data

56. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 55 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Fuel Tank Usage Data Tank full/empty times Engine/equipment start time Body Tank Input Data Tank Thermal Data Multi-Flight Monte Carlo Data

57. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 56 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Fuel Tank Usage Data Body Tank Input Data (This refers to tanks completely enclosed in the fuselage, or similar container with no direct cooling to ambient air) Is the tank in the fuselage? If yes, then what is the temperature of compartment surrounding tank? Is the tank pressurized? If yes, then what is the pressure differential? Tank Thermal Data Multi-Flight Monte Carlo Data

58. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 57 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Fuel Tank Usage Data Body Tank Input Data Tank Thermal Data Fuel temperature differentials relative to ambient Exponential time constants (define how fuel heats/cools in response to heat input) Multi-Flight Monte Carlo Data

59. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 58 FTFAM Usage - User Inputs User inputs for the program are divided into six categories Airplane Data Flight Data Fuel Tank Usage Data Body Tank Input Data Tank Thermal Data Multi-Flight Monte Carlo Data Number of Flights Random number freeze? (y/n) Warm day analysis? (y/n)

60. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 59 Program Overview - Main Calculations

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63. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 62 FTFAM Usage - Outputs Monte Carlo Flammability Analysis Table displaying the following data for each flight (<5000): Preflight ground time Flight time Ambient temperature Cruise temperature Fuel flashpoint temperature Amount of time that the tank was flammable % of flight time that the tank was flammable Table displaying warm day (ground ambient temperature > 80  F) results Chart showing a summary of the Multi-Flight Monte Carlo Analysis

64. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 63 FTFAM Usage - Outputs Single Flight Analysis Time-based and altitude-based plots of fuel temperature, TAT, LFL and UFL

65. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 64 FTFAM Usage – Modifications to the Program There are certain aspects of the program’s code that may need to be modified by the user based on acquired aircraft data. Fuel Management As discussed previously, the program assumes a constant rate of fuel usage, thereby resulting in a linear rate of change in the tank thermal time constants. It does not account for unique fuel management techniques, such as fuel transfer systems. If the linear decay model that the program uses does not adequately represent the fuel burn process, this portion of the program code can be modified. Any code modification must be shown, by flight test and a detailed analysis of the tank’s usage of fuel backed up by data, to provide an accurate representation of fuel usage by the tank.

66. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 65 FTFAM Usage – Modifications to the Program Tank Thermal Characteristics As discussed previously, the thermal behavior of the fuel tank due to its surroundings is based on calculations using the fuel temperature differential relative to ambient temperature and TAT, as well as the exponential tank thermal time constants. If flight test data and a detailed analysis of the tank’s thermal behavior shows that this method cannot yield an accurate representation of the actual fuel temperature profile, then this portion of the program code can be modified. Any code modification must be shown, by flight test and a detailed analysis of the tank’s thermal behavior backed up by data, to provide an accurate representation of the fuel temperature profile.

67. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 66 FTFAM Usage – Modifications to the Program

68. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 67 The latest version of the FTFAM and its associated User’s Manual can be downloaded at the FAA’s Fire Safety Section Website at: http://www.fire.tc.faa.gov/systems/fueltank/FTFAM.stm Any updates to either the FTFAM or its associated User’s Manual will be posted in the Federal Register

69. The Fuel Tank Flammability Assessment Method – Flammability Analysis Federal Aviation Administration 68 Example* – Heated Center Wing Tank Consider an example aircraft with a heated center wing tank (CWT). The aircraft has 4 engines, a maximum range of 10,000 nm, and is designed to fly with a typical cruise speed of mach 0.81. A flight test is flown to determine the CWT’s thermal characteristics. Click Here to Open a Spreadsheet with Sample Data and Calculations to Determine Inputs to the FTFAM The following slides show screenshots of the FTFAM displaying inputs to the program based on this data as well as a comparison of the calculated results to the sample flight data results. *note that this example, or any other contained within this presentation, do not represent any particular aircraft or fuel tank configuration, and any data provided is for demonstration purposes only.

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