Abstract: A system for generating, in an aircraft, a warning signal whenever the aircraft is on a potentially hazardous course with respect to the terrain and obstacles thereon imminently to be overflown. The system involves the computation of pull-up trajectories which the aircraft could carry out at a reference point (RP) on the current aircraft flight path (FP) forward of the aircraft current position (CP), taking account of uncertainties of at least aircraft position, which trajectories define collectively a region moving notionally forwardly of the aircraft at a spacing ahead of the aircraft which is a function of aircraft velocity and acceleration and aircraft dynamic response to pilot demands and pilot reaction time, and therefore constitutes a region which the aircraft can imminently overfly. A warning signal is produced if any point on the trajectories has a predetermined relationship with data stored in a map data storage means containing height data relating to the region defined by the trajectories.