Future air traffic concepts foresee that in unmanaged airspace, to reduce workload of air traffic controllers and the resulting constraints on capacity, the separation task will be delegated to the flight deck. Technology-driven pilot self-separation support systems have been developed that present explicit automated solutions to deal with conflicts. These systems do not offer a transparent window on the reasoning of the automation, making it difficult for pilots to judge the validity of the proposed automated solution, or come up with safe ‘good-enough’ alternatives. This thesis engaged to solve the fundamental problem of determining ‘what information’, and ‘what visual form’ would best promote pilot situation awareness (SA), to safely and effectively deal with traffic. Several prototypes for an airborne trajectory planning tool were designed and evaluated. A formative constraint-based design approach was adopted, Ecological Interface Design (EID), to create an ‘ecological’ airborne separation assistance system. The ecological approach gives priority to the worker’s environment, or ‘ecology’, focusing on how the environment imposes constraints on the worker. EID is hypothesized to improve operator SA and overall system safety when compared to normative, task-oriented, user-centered design approaches, especially in situations that were unanticipated by designers. From this thesis we conclude the following. First of all, this work has clearly shown that an ecological display, providing pilots a profound layer of information without any help of automation in terms of explicit advices, can be as safe and as effective as traditional displays that mainly present explicit automated advisories. Second, the design of ecological interfaces in domains where the abstract functions are less obvious, like the self-separation problem studied here, benefits from an incremental, evolutionary approach. Indeed, EID is not a recipe. Third, from the comparison with the more traditional design it became clear that although reducing the solution space dimension can have benefits in terms of reducing cognitive load, in the end it may lead to more cognitive load for operators to build a correct and complete mental model of the situation. Fourth, and related, although an appropriate ecologically-inspired interface can alleviate a pilot’s dependency on an explicit compelling advisory, adding dimensionality to the pilot control actions (e.g., involving more and more constraints) may render the ecological display to become too complex to be used without some sort of automated advisories. The ecological overlays developed here could be the ‘missing link’ to design a Joint Cognitive System (JCS). That is, the ecological overlays may be used to close the gap in the awareness of situations shared between automation and pilot, enabling pilots to better judge the fidelity of the proposed solution and, in case the automation fails, to come up with good-enough alternative resolutions. That is, traditional task-oriented displays and the ecological displays do not exclude each other’s use in one system. On the contrary, whereas task-oriented support may lower cognitive workload in simple standard situations through the availability of easy-to-use, automated instructions, the ecological decision support overlays show the ‘total situation’ to help the operator to become an expert and able to deal with unanticipated events. Key in this JCS design effort is to use automation as a tool to lower cognitive effort and improve decision making in such a way that it does not destroy the benefits of ecological properties of the design.