1. A method for creating a vertical trajectory profile of an aircraft by optimization of a criterion representative of a flight cost, a trajectory being computed from an ordered list of waypoints contained in the flight plan, a vertical profile associated with the trajectory being defined by a plurality of successive altitude levels to be reached by the aircraft for each level: an altitude change point between a current level (i) and a next level (i+1), a plurality of speeds observing an aircraft speed trend law on the level considered; said method comprising: loading a portion of trajectory for which the profile is to be optimized, loading at least one criterion parameter, loading at least one value representative of permitted altitude levels, initializing a first iterative computation by an initial profile (Prof 0 ), performing the first iterative computation of a profile free of altitude constraints as long as a condition dependent on the criterion is not reached, comprising the substeps: computing the criterion corresponding to the current profile using a function performing a digital integration of the trajectory, incrementing the parameters by a parametric optimization method, loading the constraint-free profile obtained as output from the first iterative computation, comprising a plurality of successive levels, and, for each level , an altitude change point and a plurality of speeds, replacing each free level of the constraint-free profile with a permitted level so as to generate an initial constrained profile comprising a plurality of permitted levels, and, for each level, an altitude change point and a plurality of speeds, loading the initial constrained profile to initialize a second iterative computation, and performing the second iterative computation of a profile in which the altitude levels to be reached remain constant, equal to the initial permitted levels of the initial constrained profile, as long as a condition dependent on the criterion is not reached, comprising the substeps: computing the criterion corresponding to the current constrained profile using a function performing a digital integration of the trajectory, incrementing the parameters by a parametric optimization method.