A terrain advisory system for an aircraft (A) comprises a processor (11) which compares terrain information, accessed from a memory means 13 according to data from a navigation means (12), with dynamic flight parameters from a monitor (14) for the aircraft (A). The processor (11) also generates a trajectory signal and a pull-up signal to indicate, on a display means (15), the current aircraft trajectory relative to a line constructed from notional pull up trajectories for the aircraft (A) which avoid a predetermined clearance height without exceeding a predetermined acceleration acting on the aircraft during a pull up manoeuvre.