Writing well-formed requirements for the successful design of sustainable long-range aircraft: a structured approach

Abstract

Several studies and research initiatives address the design of sustainable aircraft of the future, with entry-into-service between 2035 and 2050 (e.g. research projects of the Clean Aviation funding framework [1], focusing primarily on regional and short-medium range aircraft). These studies and initiatives mainly focus on new promising technologies – for example new propulsion systems based on SAF or liquid hydrogen – that once installed onboard would help the aircraft to meet sustainable, performance, circular and economical aspects. However, all these studies and initiatives can be successful only if particular care is given to the writing of well-formed requirements, which afterwards will drive the design of the sustainable aircraft. Well-formed requirements have several qualities, as clarity, unambiguity, correctness and verifiability. These requirements are correctly derived from the needs of all the stakeholders involved with the aircraft along its entire life-cycle. Additionally, these requirements do not target a specific technological solution, but match more effectively the problem space (characterized by all the conflicting stakeholder needs dealing with performance, profits, sustainably and circularity aspects) and the solution space, encompassing all the potential solutions characterizing the aircraft. Therefore, recognized and validated methods must be employed with the goal of writing well-formed requirements. This is what is being done in the EU-funded EXAELIA project (2025-2028), which aims at pre-designing and evaluating novel flying testbeds that are needed for de-risking the development of disruptive future long-range aircraft configurations and constituting technologies [2]. Given the complete list of needs that all the stakeholders have regarding the long-range aircraft, these methods prescribe how to correctly translate needs into well-formed requirements. The methods recommended by the International Council on Systems Engineering (INCOSE) [3] and extended by the research community (e.g. [4], [5]) belong to the structured approach adopted in the EXAELIA project, since they are well accepted by the engineering community (in any sector, not only in the aeronautical one). However, only a limited number of studies (e.g. [6]) addresses these methods for the definition of the requirements of future sustainable aircraft. Therefore, this work aims at increasing the awareness of these methods and promoting them by showing how they effectively improve the design process of sustainable long-range aircraft. The approach adopted in EXAELIA starts with the identification of all the stakeholders, which should encompass people or organizations who are operating the aircraft or are affected by it (e.g. passengers or citizens living close to airports). Then, the needs expressed by the various stakeholders are collected (e.g. “The society wants the flight to be carbon-neutral”). An original aspect of the EXAELIA project is the collection of the stakeholder needs addressing characteristics that in a traditional design process have lower weight than aircraft performance, operational efficiency, and regulatory compliance. These characteristics address environmental concerns, economic pressures, and evolving social expectations, hence increasing the integration of sustainability considerations from the early stages of the aircraft design process. This larger definition of stakeholder needs done in EXAELIA is described in [7] . Since stakeholder needs might be vague and unclear, they have to be transformed into requirements, which follow structures and syntactical and grammatical rules that assure that these requirements are well-formed. The need previously mentioned as example can be transformed into the aircraft-level requirement regarding the maximum CO2 emissions that the aircraft can produce in order to be carbon-neutral: “The aircraft shall emit CO2 net emissions of maximum 0 kg while in flight”. The text of this requirement contains all the elements (i.e. words) which are prescribed by [4] and [5]: a system (i.e. aircraft), a function (i.e. emit CO2 net emissions), a performance (i.e. maximum 0 Kg) and a condition (i.e. in flight). This requirement is also compliant with various syntactical and grammatical rules recommended by INCOSE [3]. In this way, it is assured that the requirement has all the qualities of well-formed requirements (e.g. clarity, unambiguity, correctness and verifiability), and it can be more effectively implemented into a technological solution. The presentation will follow up what addressed in [7] and explain the approach consisting of the structures and rules for the correct transformation of needs into requirements driving the design of future long-range aircraft. The benefits of this approach will be showcased through an application developed in the EXAELIA project. The stakeholders, needs and derived requirements of sustainable long-range aircraft designed in the EU project will be presented. References [1] Clean Aviation Joint Undertaking, "Clean Aviation," [Online]. Available: https://www.clean-aviation.eu/. [Accessed 07 April 2025]. [2] EXAELIA Consortium, "EU-Project EXAELIA: Torwards flying testbeds for novel long-range aircraft," [Online]. Available: https://exaelia.eu/. [Accessed 28th March 2025]. [3] INCOSE, "Guide for Writing Requirements, INCOSE‐TP‐2010‐006‐01," 2012. [4] R. Carson, "Implementing structured requirements to improve requirements quality," in INCOSE International Symposium, Seattle (WA), 2015. [5] L. Boggero, P. D. Ciampa and B. Nagel, "An MBSE Architectural Framework for the Agile Definition of System Stakeholders, Needs and Requirements," in AIAA Aviation Forum, Washington (US-DC), 2021. [6] L. Paletti, L. Boggero, G. Wende and B. Nagel, "Translating the concept of sustainability for the determination of requirements for the design of future aircraft," in ICAS, Florence (IT), 2024. [7] D. Markatos, "Integrating Sustainability Requirements in the Conceptual Design of Future Long-Range Aircraft: A Stakeholder-Driven Approach," in EASN, Madrid (ES), 2025 (to be published).