Fuel Tank Flammability Modeling Ivor Thomas Chief Scientific

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Fuel Tank Flammability Modeling Ivor Thomas Chief Scientific and Technical Advisor to the FAA,

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

Agenda • • • Concept Approach Assumptions Results Questions

Initial Problem • Need: To be able to assess flammability in airplane fuel tanks

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

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

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

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

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

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

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

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

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

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 –

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

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

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? ? •

Questions? ? •