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Amendment 25-125. § Amendment 25-125 Overview. New Section 25.5 - Incorporation by reference Incorporates “Fuel Tank Flammability Assessment Method User’s Manual” for § 25.981 and Appendix N Amends Section 25.981

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amendment 25 125 overview
§ Amendment 25-125 Overview
  • New Section 25.5 - Incorporation by reference
    • Incorporates “Fuel Tank Flammability Assessment Method User’s 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
guidance for amd 25 125
Guidance for amd. 25-125
  • AC 25.981-2A - Fuel Tank Flammability Reduction Means
    • Issued TBD
    • 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 will also be available in RGL in the same location as the final AC
new section 25 5 incorporation by reference
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 User’s Manual dated May 2008
    • Incorporated for § 25.981 and Appendix N
    • Provides link to manual on the FAA Technical Center Fire Safety web site
amended 25 981 overview
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
25 981 b flammability limits
§ 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 User’s 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
25 981 b regulatory text
§ 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.”

25 981 b 1 regulatory text
§ 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 User’s Manual, dated May 2008, document number DOT/FAA/AR–05/8 (incorporated by reference, see § 25.5).”

25 981 b 2 regulatory text
§ 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. ”

25 981 b 3 regulatory text
§ 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. ”

25 981 c regulatory text
§ 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. . ”

25 981 d regulatory text
§ 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.”

appendices to part 25
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
what does it mean main tanks all tanks entirely outside fuselage contour
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
what does it mean not a main tank and any part inside fuselage contour
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
flammability exposure analysis appendix n user s manual ac 25 981 2a
Flammability Exposure Analysis – Appendix N, User’s Manual, AC 25.981-2A

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
qualitative assessment conventional unheated aluminum wing tank
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
qualitative assessment minimal heating
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
monte carlo analysis
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 User’s Manual”
      • Incorporated by reference for § 25.981 and Appendix N
    • More detailed information in later presentation
monte carlo analysis background
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
monte carlo analysis flammability limits
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
monte carlo analysis flammability limits1
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