The reduction of the environmental footprint of aviation requires the development of more efficient aircraft. Emergent technologies in aircraft systems, such as more-electrical aircraft, are potential enablers for the next generation of aircraft. To support the adoption of these new technologies and to tackle the underlying integration challenges, aircraft system architectures need to be considered earlier in the aircraft design process, specifically within the conceptual design stage. To deal with the complexity and to make the system architecture development process more efficient and effective, a key enabler is to improve the representation of system architectures early in the design process. Introducing better architecture representations removes ambiguity and allows engineers to develop a shared understanding of the system. Model Based Systems Engineering (MBSE) has emerged as a systematic methodology to address complexity in systems design and overcome the drawbacks of the traditional paper based systems engineering approach used in aircraft development. This thesis investigates the use of the ARCADIA/Capella MBSE environment for the representation and specification of aircraft systems architecture in conceptual design. This thesis includes survey on the needs for system architecture representations in conceptual design. A methodology is developed within Capella to create architecture representations that are suitable for use in conceptual design. The primary flight control systems (PFCS), which by extension also includes the associated power systems, is selected to illustrate the proposed methodology. The proposed methodology consists of capturing architectural features such as interfaces, exchanges and variability. A catalog of modelling artifacts representing the various flight control actuation technologies at system level, logical and physical level has been developed. These elements can be combined to define any primary flight control system architecture. The model-based specification addresses the need to define rapidly many architecture variants for conventional and more-electrical technologies. The developed methodology is applicable to other aircraft systems. Overall, this work is an initial step towards introducing MBSE earlier in the aircraft development process thereby making it more efficient and responsive to the emerging needs of aircraft development.