Fuel Tank Flammability Modeling

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Fuel Tank Flammability Modeling

• Ivor Thomas
• Chief Scientific and Technical Advisor
• to the FAA,
• Fuel System Design
• ivor.thomas_at_faa.gov
• 425 227 1132

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Agenda

• Concept
• Approach
• Assumptions
• Results
• Questions

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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

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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).

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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

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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 1000s of cases with
  randomly selected values for each variable in
  each case.
• End result is a overall average but doesnt look
  at specific risk on any one case.

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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

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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

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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

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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.

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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

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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.

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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)

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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.

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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

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Questions??