The invention relates to a device for formulating a flight plan ensuring sufficient safety margins for a duration of a few minutes in relation to the set of flight constraints that could arise and comprising means for: detecting the surrounding moving objects (aircraft or meteorological phenomena), evaluating their type and the danger that they represent, formulating a reconfiguration flight plan ensuring a separation with these phenomena and taking best account of the constraints of the initially followed flight plan, avoiding prohibited or regulated airspaces and avoiding the surrounding relief with ad hoc operational margins.