The effect of modulationally unstable plane waves in a non-conservative cyclic and symmetric system is investigated. The main application is on vibration of bladed-disks, which are usually composed of a cyclic and periodic structure where non-linearities arise e.g. due to friction dissipation and large deformations. In this paper, we focus on unstable plane waves and their corresponding localised states through envelope dynamics. The results suggest that cyclic and symmetric non-conservative structures may experience very strong localised vibrations resembling envelope solitons if initial conditions are selected properly.