CS 25.981 Fuel tank explosion prevention

• ED Decision 2020/024/R found in: CS-25 Amdt 27 - Large Aeroplanes (Jan 2023)

CS 25.981 Fuel tank explosion prevention ED Decision 2020/024/R (a) No ignition source may be present at each point in the fuel tank or fuel tank system where catastrophic failure could occur due to ignition of fuel or vapours. This must be shown by: (1) Determining the highest temperature allowing a safe margin below the lowest expected auto-ignition temperature of the fuel in the fuel tanks. (2) Demonstrating that no temperature at each place inside each fuel tank where fuel ignition is possible will exceed the temperature determined under sub-paragraph (a)(1) of this paragraph. This must be verified under all probable operating, failure, and malfunction conditions of each component whose operation, failure, or malfunction could increase the temperature inside the tank. (3) Except for the ignition sources due to lightning addressed by CS 25.954, demonstrating that an ignition source could not result from each single failure, from each single failure in combination with each latent failure condition not shown to be extremely remote, and from all combinations of failures not shown to be extremely improbable, taking into account the effects of manufacturing variability, ageing, wear, corrosion, and likely damage. (b) Fuel tank flammability (1) To the extent practicable, design precautions must be taken to prevent the likelihood of flammable vapours within the fuel tanks by limiting heat and energy transfer (See [AMC 25.981(b)(1)](#_DxCrossRefBm1709821467)). (2) Except as provided in sub-paragraph (4) of this paragraph, no fuel tank Fleet Average Flammability Exposure level may exceed the greater of: (i) three percent, or (ii) the exposure achieved in a fuel tank within the wing of the aeroplane model being evaluated. If the wing is not a conventional unheated aluminium wing, the analysis must be based on an assumed Equivalent Conventional Unheated Aluminium Wing (see [AMC 25.981(b)(2)](#_DxCrossRefBm1709821466)). The Fleet Average Flammability Exposure is determined in accordance with appendix N of CS-25. (3) Any active Flammability Reduction means introduced to allow compliance with sub-paragraph (2) must meet appendix M of CS-25. (4) Sub-Paragraph (2) does not apply to a fuel tank if following an ignition of fuel vapours within that fuel tank the aeroplane remains capable of continued safe flight and landing. (c) Reserved. (d) To protect design features that prevent catastrophic ignition sources within the fuel tank or fuel tank system according to subparagraph (a) of this paragraph, and to prevent increasing the flammability exposure of the tanks above that permitted in subparagraph (b) of this paragraph, the type design must include critical design configuration control limitations (CDCCLs) identifying those features and providing instructions on how to protect them. To ensure the continued effectiveness of those features, and prevent degradation of the performance and reliability of any means provided according to subparagraphs (a) or (b) of this paragraph, the type design must also include the necessary inspection and test procedures, intervals between repetitive inspections and tests, and mandatory replacement times for those features. The applicant must include information required by this subparagraph in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by [CS 25.1529](#_DxCrossRefBm1709821031). The type design must also include visible means of identifying the critical features of the design in areas of the aeroplane where foreseeable maintenance actions, repairs, or alterations may compromise the CDCCLs. [Amdt 25/1] [Amdt 25/6] [Amdt 25/9] [Amdt 25/18] [Amdt 25/26]