A method and system for determining in real time a vertical trajectory of an aircraft is provided. The method includes a step for providing an initial vertical trajectory comprising an initial phase for changing flight level according to a first slope, between a first point at a first altitude, and a second point at a second altitude, at least one step for modifying the vertical trajectory, comprising a phase for detecting a triggering element when the aircraft is at the first altitude, when said triggering element is detected, and a phase for determining a modified vertical trajectory, said modified vertical trajectory comprising a modified phase for changing flight level according to a second predefined slope, from a modified point at said first altitude, distinct from said first point, to said second altitude.