Disclosed is a method and device for assisting guidance of an aircraft. A flight management system is used to determine guidance instructions that include speed instructions having at least one flight time constraint to guide the aircraft to a check-point. An accuracy level for complying with the flight time constraint and at least one error level for at least one flight parameter are generated by an accuracy generating unit. An operating margin for at least one second parameter is determined from the determined accuracy and error levels, with the operating margin being representative of air speed criterion values of the aircraft. A range determining unit is used to determine whether the range of air speed criterion values is within an air speed envelope of the aircraft, and the speed instructions are modified when the range of air speed criterion values is not within the air speed envelope.