Airborne wind energy is an emerging technology that uses tethered unmanned aerial vehicles for harvesting wind energy at altitudes higher than conventional towered wind turbines. To make the technology competitive to other renewable energy technologies an automatic control system is required that allows autonomously operating the system throughout all phases of flight. In this study a modular control system is presented, adapting the underlying kinematic and dynamic framework from conventional aerospace terminology and applying this to tethered crosswind flight with varying tether length. The high level control strategy in form of a state machine as well as the cascaded flight control structure consisting of path-following guidance and control, attitude and rate loop is presented along with the winch controller. The present work is a first step towards a methodology for the systematic development of reliable and high-performance control solutions for airborne wind energy systems. Models for the airborne system, ground station, as well as the tether connecting the ground system with the airframe will be presented. Results from a simulation study in a realistic wind field will be used to demonstrate the feasibility of the proposed concept and to identify particularly challenging situations in the operational envelope.