An aid method for piloting an aircraft including acquiring an instruction intended to alter the flight plan, determining an applicability period of the instruction made up of a plurality of applicability moments of the instruction in which the application of the instruction is compatible with the operational capabilities of the aircraft and with the constraints of the flight plan, for each applicability moment of the instruction, determining an altered flight plan in case of application of the instruction at this applicability moment and calculating values of the operational parameters associated with this flight plan, and selecting an optimal applicability moment based on values of the operational parameters associated with the altered flight plan and operational parameters associated with the current flight plan.