Methods and systems for aircraft guidance

Methods and systems for guiding an aircraft are disclosed herein. An example method includes determining, via a processor, a difference between an estimated time of arrival and a required time of arrival of an aircraft. The example method also includes determining if the difference exceeds a threshold time. The example method further includes determining a deviation between a predicted four-dimensional flight trajectory of the aircraft and a measured four-dimensional flight trajectory of the aircraft during flight if the difference does not exceed the threshold time. The example method also includes generating a speed command to reduce the deviation and updating a flight plan of the aircraft based on the speed command.