Development of a FAA Special Condition for Flammability Reduction Systems

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Development of a FAA Special Condition for
Flammability Reduction Systems

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

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Background

• With the development by the FAA of a concept for
  a practical inerting system, the Boeing Company
  elected to move forward with the development and
  certification of such a system on a 747.
• The FAA and EASA determined that the current
  FARs did not address such a system adequately
  and therefore harmonized Special Conditions (SC)
  were needed.
• Special conditions were created jointly with
  EASA, TCCA and FAA.

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Special Conditions Objectives

• As the Boeing Company was moving forward with a
  Nitrogen Generation System for the 747, the FAA
  and EASA had to address a number of issues in the
  SC,
• What level of flammability reduction was needed
• How to define and determine tank flammability
• How to determine a satisfactory O2 level to be
  considered Inert
• How to set standards that were performance based
  and did not impose a single solution
• How to address any new hazards that inerting may
  present
• Ensuring harmonization of the special conditions

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Fundamental Features of the SC

• Objectives 1) Reduce Flammability to an
  acceptable level, 2) Address warm day specific
  risk
• Fleet average exposure less than 3 of
  operational time - all flight phases combined
• 3 divided approximately equally into
• 1.5 for system performance limitations
• 1.5 for times when system is unavailable
• Fleet exposure on days over 80 deg F when system
  is available
• For ground and climb phases, tank flammable less
  than 3 of operational time

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Fundamental Features of the SC

• Fleet Average Exposure less than 3 of
  Operational Time
• This provides a means of regulating the fleet
  average exposure to a level commensurate with the
  historical level of unheated wing tanks.
• The division of the 3 into two approximately
  equal sub-requirements is intended to ensure that
  the flammability requirement is not degraded by
  system unreliability, while allowing the design
  to be focused on normal operation rather than
  being designed for a worst case flight.
• One 1.5 sub-requirement allows the design to be
  sized for normal operation and extreme cases,
  such as emergency descents, can be included in
  this allowance.
• The second 1.5 recognizes that the system is
  intended to be a single thread system and there
  will be occasions when the system is unavailable,
  both when the system has failed but has not been
  recognized as failed, and occasions when the
  system is known to be inoperative
• The design must ensure the overall requirements
  are met, by addressing performance, reliability
  and system health monitoring

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Fundamental Features of the SC

• Fleet exposure on warm days - over 80 deg F when
  system is functional
• On the ground, tank flammable less than 3 of
  time on ground
• In climb, tank flammable less than 3 of time in
  climb
• This requirement was included in the SC to ensure
  that the system addresses the high flammability
  exposure of hot days on the ground and in climb.
• In principle, a system could meet the average 3
  requirement only, by addressing the higher
  altitude conditions and by being very reliable.
• This is considered unacceptable because this
  system would not address the conditions that were
  present on the most recent three fuel tank
  explosion accidents.
• The 80 Deg F warm day conditions ensure that
  the system protects the airplane on warm days,
  again accepting the concept of a single thread
  system.

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Fundamental Features of the SC

• Use of the ARAC Monte Carlo model approach is
  required
• SC limits certain parameters for consistency
• Temperature variation on ground and cruise
• Overnight temperature drop
• Fuel Flash point variation
• Flammability envelope
• based on 1 joule spark (Equivalent to envelope
  used by ARAC)
• Applicant can use airplane specific data where
  appropriate and substantiated by data
• Airplane/Engine parameters
• Mission distribution

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Fundamental Features of the SC

• The SC is performance based, and not limited to a
  specific design solution.
• The fundamental requirement is a level of
  flammability as determined by a standard
  approach.
• Alternative solutions can be addressed by showing
  that the flammability targets are met.

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Additional Features of the SC

• Although the SC is performance based, inerting is
  one way of satisfying the requirements.
• The tank is considered inert if the O2 level is
  below 12 for altitudes from Sea Level to 10,000
  ft, and then increasing linearly to below 14.5
  at 40000 ft.
• Linear extrapolation above 40,000 ft is
  acceptable.
• Thus the tank is considered non-flammable if
  either the tank is outside the flammability
  envelope for the fuel on board, or the O2 level
  satisfies the inert definition above.

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Additional Features of the SC

• In the descent case, localized concentrations
  above the inert level are allowed.
• This is judged to be acceptable based on the very
  lean conditions of the entering air (no
  hydrocarbons present and cold temperature) and
  the time needed to mix the incoming air with the
  existing tank ullage.
• Under these conditions, the tank is considered
  non-flammable during descent provided the tank
  was non-flammable at the entry into the descent.
• If the tank is above 12 O2 at touch down, the
  time after touch down when the tank is flammable
  is counted towards the determination of
  flammability exposure.

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Additional Features of the SC

• In the case of a multi-bay tank, i.e. one where
  individual bays of the tank are separated from
  each other by walls with very little air or fuel
  communication,
• Conditions can occur where one or more bays may
  be at a higher O2 level than the rest of the
  bays.
• Under these conditions, the tank is considered
  flammable if the O2 level is above the inert
  level in any bay and the temperature of the tank
  is in the flammable range

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Harmonization

• The FAA, EASA and TCCA have worked closely in
  developing the harmonized SC,
• The FAA proposed SC and the EASA equivalent were
  developed in harmony with TCCA and released for
  comment as virtually identical text.
• The comment periods for both were the same, and
  FAA and EASA have reviewed, and disposed of,
  comments together.
• The final SC is harmonized with EASA.

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Summary

• The SC is performance based with flammability
  levels intended to provide a significant
  improvement in safety.
• Inerting is one acceptable means of complying
  with the SC and is addressed in the SC, but other
  systems may be proposed to meet the SC.

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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