Amendment 25-125

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Amendment 25-125
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Amendment 25-125 Overview

• New Section 25.5 - Incorporation by reference
• Incorporates Fuel Tank Flammability Assessment
  Method Users Manual for 25.981 and Appendix N
• Amends Section 25.981
• Replaces amd. 25-102 minimize fuel tank
  flammability standard with limits for all fuel
  tank types
• For main tanks and tanks located entirely outside
  the fuselage contour, requires 3 Fleet Average
  Flammability Exposure or (equivalent)
  conventional unheated aluminum wing tank,
  whichever is greater
• Other than a main tank and any part located
  inside fuselage, requires flammability meet
  Appendix M requirements
• 3 Fleet Average Flammability Exposure and 3
  warm day
• Adds fuel tank flammability airworthiness
  limitation requirement
• New Appendices to Part 25
• Appendix M Fuel Tank System Flammability
  Reduction Means
• Appendix N - Fuel Tank Flammability Exposure and
  Reliability Analysis

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Guidance for amd. 25-125

• AC 25.981-2A - Fuel Tank Flammability Reduction
  Means
• Issued 9/19/2008
• Available in FAA Regulatory and Guidance Library
  (RGL) web site
• http//rgl.faa.gov/Regulatory_and_Guidance_Library
  /rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet
• The Public Comment Disposition Table is also be
  available in RGL in the same location as the AC

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New Section 25.5 - Incorporation by reference

• New section provides common location for
  incorporating documents by reference
• Provides process for revising documents by
  publishing a notice in the Federal Register
• 25.5(1)(b) Incorporates Fuel Tank Flammability
  Assessment Method Users Manual dated May 2008
• Incorporated for 25.981 and Appendix N
• Provides link to manual on the FAA Technical
  Center Fire Safety web site

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Amended 25.981 - Overview

• 25.981 - Fuel tank explosion prevention
• Replaces Fuel tank ignition prevention
• 25.981(b) provides new fuel tank flammability
  requirements
• 25.981(c) retains previous option to mitigate
  effects of fuel vapor ignition (e.g. foam)
• 25.981(d) adds fuel tank flammability
  airworthiness limitation requirement

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25.981(b) - Flammability Limits

• 25.981(b) - New fuel tank flammability
  requirements
• 25.981(b) Flammability requirement - Main
  tanks and tanks located entirely outside the
  fuselage contour
• 25.981(b)(1) Requires flammability exposure
  be determined per Appendix N and Users Manual
• 25.981(b)(2) Flammability requirement - Fuel
  tanks other than main tanks meet Appendix M
  flammability requirements if any portion is
  located within fuselage contour
• 25.981(b)(3) Definition of certain terms used

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25.981(b) Regulatory Text

• Except as provided in paragraphs (b)(2) and (c)
  of this section, no fuel tank Fleet Average
  Flammability Exposure on an airplane may exceed
  three percent of the Flammability Exposure
  Evaluation Time (FEET) as defined in Appendix N
  of this part, or that of a fuel tank within the
  wing of the airplane model being evaluated,
  whichever is greater. If the wing is not a
  conventional unheated aluminum wing, the analysis
  must be based on an assumed Equivalent
  Conventional Unheated Aluminum Wing Tank.

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25.981(b)(1) Regulatory Text

• Fleet Average Flammability Exposure is
  determined in accordance with Appendix N of this
  part. The assessment must be done in accordance
  with the methods and procedures set forth in the
  Fuel Tank Flammability Assessment Method Users
  Manual, dated May 2008, document number
  DOT/FAA/AR05/8 (incorporated by reference, see
  25.5).

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25.981(b)(2) Regulatory Text

• Any fuel tank other than a main fuel tank on an
  airplane must meet the flammability exposure
  criteria of Appendix M to this part if any
  portion of the tank is located within the
  fuselage contour.

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25.981(b)(3) Regulatory Text

• As used in this paragraph,
• (i) Equivalent Conventional Unheated Aluminum
  Wing Tank is an integral tank in an unheated
  semi-monocoque aluminum wing of a subsonic
  airplane that is equivalent in aerodynamic
  performance, structural capability, fuel tank
  capacity and tank configuration to the designed
  wing.
• (ii) Fleet Average Flammability Exposure is
  defined in Appendix N to this part and means the
  percentage of time each fuel tank ullage is
  flammable for a fleet of an airplane type
  operating over the range of flight lengths.
• (iii) Main Fuel Tank means a fuel tank that feeds
  fuel directly into one or more engines and holds
  required fuel reserves continually throughout
  each flight.

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25.981(c) Regulatory Text

• Paragraph (b) of this section does not apply to
  a fuel tank if means are provided to mitigate the
  effects of an ignition of fuel vapors within that
  fuel tank such that no damage caused by an
  ignition will prevent continued safe flight and
  landing. .

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25.981(d) Regulatory Text

• Critical design configuration control
  limitations (CDCCL), inspections, or other
  procedures must be established, as necessary, to
  prevent development of ignition sources within
  the fuel tank system pursuant to paragraph (a) of
  this section, to prevent increasing the
  flammability exposure of the tanks above that
  permitted under paragraph (b) of this section,
  and to prevent degradation of the performance and
  reliability of any means provided according to
  paragraphs (a) or (c) of this section. These
  CDCCL, inspections, and procedures must be
  included in the Airworthiness Limitations section
  of the instructions for continued airworthiness
  required by 25.1529. Visible means of
  identifying critical features of the design must
  be placed in areas of the airplane where
  foreseeable maintenance actions, repairs, or
  alterations may compromise the critical design
  configuration control limitations (e.g.,
  color-coding of wire to identify separation
  limitation). These visible means must also be
  identified as CDCCL.

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Appendices to Part 25

• Appendix M - Fuel Tank System Flammability
  Reduction Means
• Flammability Exposure Requirements
• Reliability indications and maintenance access
• Airworthiness limitations and procedures
• Appendix N - Fuel Tank Flammability Exposure and
  Reliability Analysis
• Defines analysis method for showing compliance to
  25.981(b) and Appendix M
• Includes definitions, fixed and variable
  parameters, requirements if flammability
  reduction means (FRM) is used

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What does it mean? Main tanks all tanks
entirely outside fuselage contour

• Fleet Average Flammability Exposure can not
  exceed (whichever is greater)
• 3 of the Flammability Exposure Evaluation Time
  (FEET) as defined in Appendix N or,
• The Fleet Average Flammability Exposure of a wing
  tank on the airplane model being evaluated.
• If airplane model does not use conventional
  unheated aluminum wing, then requires an analysis
  based on an Equivalent Conventional Unheated
  Aluminum Wing Tank
• Wing tanks, including composite wing tanks, are
  required to meet fleet average flammability
  exposure limit established above
• All fuel tanks required to meet amd. 25-125 will
  require flammability airworthiness limitations,
  including CDCCL

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What does it mean? Not a main tank and any part
inside fuselage contour

• Fleet Average Flammability Exposure can not
  exceed (per Appendix M)
• 3 and,
• 3 warm day limit for either ground or
  takeoff/climb phases
• If FRM is used, additional Fleet Average
  Flammability Exposure requirements apply
• 1.8 limit for times FRM is operational and tank
  is not inert and is flammable, and
• 1.8 limit for times FRM is inoperative and tank
  is flammable
• All fuel tanks required to meet amd. 25-125 will
  require flammability airworthiness limitations
• Includes CDCCLs
• If FRM is used, additional FRM limitations apply

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Flammability Exposure Analysis Appendix N,
Users Manual, AC 25.981-1A

• Appendix N, N25.1(a), provides two means of
  conducting fuel tank flammability exposure
  analysis
• Qualitative Method - If the fuel tank is
• Installed in an aluminum wing, and
• Substantiated to be a conventional unheated wing
  tank
• Monte Carlo Model - All other fuel tanks

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Qualitative Assessment Conventional Unheated
Aluminum Wing Tank

• Characteristics of a conventional unheated
  (aluminum) wing tank are described in AC
  25.981-2A
• Conventional aluminum structure
• Integral tank of a subsonic transport airplane
  wing
• Minimal heating from airplane systems or other
  fuel tanks
• Cooled by ambient airflow during flight
• Not insulated
• Large aerodynamic surface area exposed to outside
  air

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Qualitative Assessment Minimal heating

• Heat sources that may be minimal
• Heat exchanges with little or no affect on fuel
  temperature
• Thermal anti-ice systems
• Thermal anti-ice blankets
• Heat sources that may not be minimal
• Heat exchangers with significant affect on fuel
  temperature
• Adjacent heated fuel tanks
• Transfer of fuel from a warmer tank
• Adjacent air conditioning equipment

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Monte Carlo Analysis

• When to use
• Wing tank does not meet the qualitative
  assessment criteria
• When required to demonstrating a tank meets a
  specific flammability exposure limit (value)
• How to perform
• Appendix N to part 25
• Fuel Tank Flammability Assessment Method Users
  Manual
• Incorporated by reference for 25.981 and
  Appendix N
• More detailed information in later presentation

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Monte Carlo Analysis - Background

• Comparative analysis tool
• Developed by 1998 ARAC working group to compare
  flammability reduction methods
• Simplified flammability analysis
• Ullage flammability based on bulk fuel
  temperature and pressure altitude
• Condensation not considered
• Mass Loading not considered
• Standardized by Special Conditions Amd. 25-125
• Certain standardized parameters used for all
  airplane models
• Airplane specific parameters used where necessary

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Monte Carlo Analysis Flammability Limits

• Numerical flammability limits based on
  standardized Monte Carlo Analysis method
• Flammability limits agreed during development of
  initial Harmonized FRM special conditions
• 3 Fleet Average Limit
• Basis is Monte Carlo analysis of Generic
  Conventional Unheated Aluminum Wing Tank
• Basis for comparison of flammability of other
  fuel tanks using Monte Carlo analysis
• Lower limit if allowed for condensation in
  conventional unheated aluminum wing tank

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Monte Carlo Analysis Flammability Limits

• 3 Warm Day Limit
• Developed to address warm conditions during most
  recent fuel tanks explosions (Jet A fuel)
• 1.8 Performance or Reliability Limit
• Balances requirements for reliable design and
  performance design

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Questions -

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