A method for determining a flight path for an aircraft system, for example an unmanned aircraft system (UAS) comprises analysing an intensity map relating to a three dimensional space. The intensity map comprises an array of voxels, each voxel defining a volume in the three dimensional space, and each voxel having a related traffic intensity value based on historical flight data through that voxel. The method comprises determining a probability of an encounter for a preferred flight path between a start point and an end point via one or more voxels in the three dimensional space, based on traffic intensity values of the one or more voxels along the preferred flight path. The preferred flight path is selected if the probability of encounter is less than a first threshold value.