We present an immersive environment where Virtual Reality (VR) is used to visualize the performance of a fleet of aircraft engines. Our virtual environment uses 3D geometric computer-aided design (CAD) models of the engines paired with performance maps that characterize their nominal working condition. These maps plot pressure ratio and efficiency as a function of shaft speed and inlet flow capacity for the numerous engine sub-systems. Superimposed on these maps is the true performance of each engine, obtained through real-time sensors. In this bespoke virtual space, an engineer can rapidly analyze the health of different engine sub-systems across the fleet within seconds. One of the key elements of such a system is the selection of an appropriate interaction technique. In this paper we explore the potential of interaction methods supported by a combination of gaze-tracking and hand-tracking achieved via an additional sensor attached to the front of the VR headset, with no need for the user to hold a controller. We report on an observational study with a small number of domain-experts to identify usability problems, spot potential improvements, and gain insights into our design interaction capabilities. The study allows us to trim the design space and to guide further design efforts in this area. We also analyze qualitative feedback provided by the end-users and discuss the lessons learned during the design, implementation, verification and validation of the system.EPSRC (EPSRC-1788814); Cambridge European and Trinity Hall; Cambridge Philosophical Society Research Studentship; Tsinghua Academic Fund for Undergraduate Overseas Studies; und from Tsien Excellence in Engineering Program; Wave 1 of The UKRI Strategic Priorities Fund under the EPSRC Grant EP/T001569/1, particularly the ``Digital Twinning in Aeronautics'' theme within that grant & The Alan Turing Institut
