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The Future Flight Standards Hub is designed to accelerate the development of Future Flight demonstrators and wider industry innovation based on emerging best practice, knowledge sharing, and standards support.

  • BSI Flex 1903 v2:2024-06 community preview

    This is an interactive version 2 of BSI Flex 1903. The aim of the standard is to create a lexicon of standard terms and acronyms used in this sector. Please note, content is still undergoing internal review and there may be minor changes at full publication.

    We welcome your engagement and comments on this version to help inform the development of the standard. We would also welcome general feedback including what terms could be considered for future versions and these can be made through our dedicated feedback page .

state of function (SOF)



  • Definition

    indication of the capability to provide accessible energy for safe flight and landing before the corresponding end point condition of an energy storage system (ESS) (3.1.69) is met


    NOTE 1 Provides a comparison of continuously observed measurements from battery sensors to the expected values during the flight.

    NOTE 2 Utilizes multiple inputs; examples include battery cell (3.1.26) state of charge (SOC) (3.1.126) cell temperature and cell current.

    NOTE 3 SOF is one of the measures used to provide an indication to crew of the current status of the energy storage system (ESS) (3.1.69) in order to allow an understanding of the remaining flight time and/or flight range.


    SOURCE: EUROCAE, ED-289, Guidance on determination of accessible energy in battery systems for eVTOL applications [25]

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