Electronic devices and methods for optimizing the vertical profile to be flown by an aircraft during the cruise phase of a flight. Based on continuously updated information about the aircraft's weight and the atmospheric wind and temperature, the method provides an optimal sequence of climbs and/or descents along the flight path during the cruise phase. Following the step climb/descent profile proposed by the method results in the most cost-optimal flight (if a cost index was selected) or in the most fuel-efficient flight (if the long-range cruise mode was selected). The method may be implemented in the flight management computer or any other electronic data processing device that can access the required information to perform the calculations.