An auxiliary power unit (APU) control system for an aircraft , the APU control system including an APU configured to be drivingly coupled to one or more generators , the APU control system comprising: one or more processors ; and a memory coupled to the one or more processors, the memory storing data comprising a database and program code that, when executed by the one or more processors, causes the APU control system to: acquire one or more ambient signals indicative of an air density value and one or more power signals indicative of a specific amount of power generated by the APU; determine a first variable rotational speed of the APU based on the air density value; instruct the APU to operate at the first variable rotational speed, wherein the APU continues to generate the specific amount of power when operating at the first variable rotational speed; after instructing the APU to operate at the first variable rotational speed, acquire an electrical load signal , wherein the electrical load signal is indicative of an electrical load demand by the one or more generators; determine a second variable rotational speed of the APU based on the electrical load demand by the one or more generators; and instruct the APU to operate at the second variable rotational speed. one or more processors ; and a memory coupled to the one or more processors, the memory storing data comprising a database and program code that, when executed by the one or more processors, causes the APU control system to: acquire one or more ambient signals indicative of an air density value and one or more power signals indicative of a specific amount of power generated by the APU; determine a first variable rotational speed of the APU based on the air density value; instruct the APU to operate at the first variable rotational speed, wherein the APU continues to generate the specific amount of power when operating at the first variable rotational speed; after instructing the APU to operate at the first variable rotational speed, acquire an electrical load signal , wherein the electrical load signal is indicative of an electrical load demand by the one or more generators; determine a second variable rotational speed of the APU based on the electrical load demand by the one or more generators; and instruct the APU to operate at the second variable rotational speed. acquire one or more ambient signals indicative of an air density value and one or more power signals indicative of a specific amount of power generated by the APU; determine a first variable rotational speed of the APU based on the air density value; instruct the APU to operate at the first variable rotational speed, wherein the APU continues to generate the specific amount of power when operating at the first variable rotational speed; after instructing the APU to operate at the first variable rotational speed, acquire an electrical load signal , wherein the electrical load signal is indicative of an electrical load demand by the one or more generators; determine a second variable rotational speed of the APU based on the electrical load demand by the one or more generators; and instruct the APU to operate at the second variable rotational speed. <EOS>