An aircraft mission computing system, comprising a computing engine to compute trajectories of the aircraft during a mission, comprising: - a initializing module of specifications of a mission including at least a geographical point of origin, a geographical destination point, and operational mission specifications; - a weather context recovery module to recover a weather context in the airspace between the geographical point of origin and the geographical destination point; - a trajectories computing module able to compute a mission trajectory between the geographical point of origin and the geographical destination point as a function of airplane performances and as a function of operational mission specifications, caracterised in that the system comprises an airplane performances determining module able to determine the airplane performances based on the operational mission specifications and based on the weather context, and in that the trajectories computing module is configured to compute a plurality of iso-movement curves on several flight levels, in particular a plurality of isochronous curves and/or a plurality of fuel iso-consumption curves from at least one point of the trajectory and is able to determine each point of the trajectory on a iso-movement curve relative to another point of the trajectory, in which the computing module is configured to determine the iso-movement curves corresponding to a plurality of flight levels from an initial point to a first flight level change curve and to define at least a first trajectory segment for each flight level, then the iso-movement curves from the first flight level change curve for a plurality of flight levels to another flight level change curve and to define at least a second trajectory segment for each flight level, then to define at least one trajectory by combining successive trajectories segments chosen from among the trajectories segments defined for each plurality of flight levels. - a initializing module of specifications of a mission including at least a geographical point of origin, a geographical destination point, and operational mission specifications; - a weather context recovery module to recover a weather context in the airspace between the geographical point of origin and the geographical destination point; - a trajectories computing module able to compute a mission trajectory between the geographical point of origin and the geographical destination point as a function of airplane performances and as a function of operational mission specifications, caracterised in that the system comprises an airplane performances determining module able to determine the airplane performances based on the operational mission specifications and based on the weather context, and in that the trajectories computing module is configured to compute a plurality of iso-movement curves on several flight levels, in particular a plurality of isochronous curves and/or a plurality of fuel iso-consumption curves from at least one point of the trajectory and is able to determine each point of the trajectory on a iso-movement curve relative to another point of the trajectory, in which the computing module is configured to determine the iso-movement curves corresponding to a plurality of flight levels from an initial point to a first flight level change curve and to define at least a first trajectory segment for each flight level, then the iso-movement curves from the first flight level change curve for a plurality of flight levels to another flight level change curve and to define at least a second trajectory segment for each flight level, then to define at least one trajectory by combining successive trajectories segments chosen from among the trajectories segments defined for each plurality of flight levels. caracterised in that the system comprises an airplane performances determining module able to determine the airplane performances based on the operational mission specifications and based on the weather context, and in that the trajectories computing module is configured to compute a plurality of iso-movement curves on several flight levels, in particular a plurality of isochronous curves and/or a plurality of fuel iso-consumption curves from at least one point of the trajectory and is able to determine each point of the trajectory on a iso-movement curve relative to another point of the trajectory, in which the computing module is configured to determine the iso-movement curves corresponding to a plurality of flight levels from an initial point to a first flight level change curve and to define at least a first trajectory segment for each flight level, then the iso-movement curves from the first flight level change curve for a plurality of flight levels to another flight level change curve and to define at least a second trajectory segment for each flight level, then to define at least one trajectory by combining successive trajectories segments chosen from among the trajectories segments defined for each plurality of flight levels.