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Fuel Tank Flammability Modeling Ivor Thomas Chief Scientific and Technical Advisor to the FAA, Fuel System Design ivor. [email protected] gov 425 227 1132
Agenda • • • Concept Approach Assumptions Results Questions
Initial Problem • Need: To be able to assess flammability in airplane fuel tanks so that safety enhancements could be assessed against each other. • Problem: Flights all over the world create widely varying conditions and times when a tank may be flammable, but FAA needed to assess the overall safety benefits of an enhancement
Concept • Create a computer model to: – Assess tank flammability for a large number of flights throughout the world, – Assess the impact of any enhancements on reducing overall flammability, – look at risks in specific conditions, – (and make it simple enough to run quickly).
Approach • Monte Carlo technique to create several thousand flights in worldwide atmospheric conditions with critical variables such as flash point of the fuel also varying to represent the real world. • This approach required several sub-models – – – The airplane performance The tank thermal response The atmosphere The fuel The system enhancement proposed
Monte Carlo Analysis • Technique to allow a statistical analysis of a problem with a number of independent variables • Technique uses known distribution probabilities for variables and runs 1000’s of cases with randomly selected values for each variable in each case. • End result is a overall average but doesn’t look at specific risk on any one case.
Airplane Performance • A simple airplane performance model was developed to allow various airplanes to be studied, which included – – Time on the ground Fuel load Climb: Time/Speed schedule Cruise: Alt. And Mn. including step climbs on longer flights – Descent and Landing: Time/Speed schedule in Descent – Mission Length Distribution
Tank Thermal Response • Tank treated as simple object with thermal response characteristics determined from separate thermal modeling or flight test. – Characteristics defined by: • Exponential time constant for full and empty conditions both ground and flight • Equilibrium temperature the tank would reach (given enough time) relative to total air temperature for both ground and flight
Atmospheric model • Any flight uses two inputs, – Ground ambient, and – Ambient temperature above the Tropopause. • For any given flight, the two values are picked randomly to match the known world temperature distribution
Atmospheric model • The Temperature Profile versus Altitude is the determined, using a standard lapse rate to the tropopause, and constant above, with a temperature inversion effect if the ground ambient is below 00 F.
Fuel Variability and Flammability • The FAA has surveyed the fuels being used by the fleet and determined the flash point range and distribution, • Fuel Air Ratio at the Flash Point has been measured for a number of these fuels, and this has been used to correlate Flammability range to Flash Point
Flammability Assessment • An Computer model was created to integrate all the factors discussed to predict fuel tank flammability – Model can run one flight to look at specific risk, or – Model can run several thousand flights to determine fleet average fuel tank flammability exposure.
Potential Mitigating Effects • The model can be used to assess mitigating systems – Reduced heat flow to the tank – Increased heat flow out of the tank – Fuel Tank Inerting • Ground Only • In-flight • In-Flight with limitations (e. g. Low nitrogen flow in Descent)
Conclusions • The Flammability Exposure model has given the FAA and industry a common tool to assess fuel tank flammability and to evaluate potential mitigating actions.
Open Issues • Flammability limits with altitude and with various levels of Nitrogen in the ullage – FAA testing in progress • Flammability Limits with very low fuel quantities and time dependent effects of non -equilibrium conditions – FAA /Rutgers University work in progress
Questions? ? •