Development of a Model to Calculate Fuel Tank Flammability

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Development of a Model to Calculate Fuel Tank
Flammability

• Ivor Thomas
• Consultant to FAA
• 1-425 455 1807
• fuelsguy_at_msn.com

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Presentation Agenda

• Concept
• Approach
• Assumptions
• Results
• Conclusions

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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 any 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 large amount of data, but
  typically the fleet average exposure is used as a
  reference for the tank in question.
• Specific high risk areas can be examined, such as
  days above 80 deg F

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Airplane Performance

• An 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

• A 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.
• Model can determine flammability for specific
  ranges of conditions

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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. High nitrogen
  flow in Descent)
• Other Flammability Reduction methods

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Potential Mitigating Effects (Continued)

• The model must be modified by the user to
  represent the reduction methodology.
• For any system the model computes if the tank
  flammable every one minute. The user can add
  instructions to overlay this and change a
  flammable count to a non-flammable count if
  the system is effective at that point in the
  flight.

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Potential Mitigating Effects (Continued)

• For Example,
• A ground based inerting system is effective once
  the tank is flooded with N2, and will stay that
  way until the tank breathes in sufficient air to
  dilute the N2.
• IF the tank is empty, the tank remaining inert
  until the end of cruise,
• IF the tank is full, the tank will become
  un-inert early in cruise
• Algorithms added to the model can compute this
  and reduce flammability appropriately.

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Current Status

• The Flammability Exposure model has given the FAA
  and industry a common tool to assess fuel tank
  flammability and to evaluate potential mitigating
  actions.
• The 747 proposed Special Condition uses the model
  to define the current and expected flammability
  exposure when an NGS or other flammability
  reduction method is added.

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Current Status

• FAA is drafting a revised AC25.981-2 with the
  assistance of AIA to reflect this approach.

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Model Enhancements

• FAA model is generic and must be made specific
  for a given airplane.
• Thermal models
• Fleet mission distribution
• Daily flight distribution effects
• Mitigation System effects

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Cautions

• The original model was developed by ARAC I to
  assess the correlation of flammability to the
  fleet accident history
• ARAC I concluded that low flammability tanks,
  (fleet average exposure below 7) could be
  considered acceptable but high flammability tanks
  needed t o be addressed
• As the model is refined and modified the 7 may
  need to be examined for its validity.

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Conclusions

• FAA has provided a tool to allow the industry to
  assess flammability in fuel tanks and to evaluate
  the effectiveness of mitigating actions.
• Enhancements could be incorporated but we must
  recognize the model is a comparative tool not an
  absolute predictor of flammability.
• The baseline Low Flammability level of 7 needs
  to be examined carefully if too many enhancements
  to the model are made.

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The Fourth Triennial International Aircraft Fire
and Cabin Safety Research Conference
The Fourth Triennial International Aircraft Fire
and Cabin Safety Research Conference