Constant speed drive to control variable APU speed and constant generator output frequency

A method of controlling an aircraft hydraulic control system, the method comprising:

• driving an auxiliary power unit (APU) drive shaft at a first rotational speed during a first condition using an APU and driving the APU drive shaft at a second rotational speed greater than the first during a second condition using the APU;
• transferring one of a first rotational power to a constant speed drive (CSD) unit via the APU drive shaft based on the first rotational speed and a second rotational power to the CSD unit via the APU drive shaft based on the second rotational speed;
• converting the first rotational power into a constant rotational power using the CSD to drive a generator drive shaft coupled to a generator at a constant target rotational speed, and converting the second rotation power into the constant rotational power using the CSD to drive the generator drive shaft coupled to the generator at the constant target rotational speed;
• and
• producing an alternating current (AC) voltage having a target frequency using the generator in response to rotation of the generator shaft at the constant target rotational speed,
• wherein the first condition is a first altitude range at which the APU drives the APU drive shaft at the first rotational speed falling within a first speed range which ranges from about 20,429 RPM to about 22,000 RPM, and the second condition is a second altitude at which the APU drives the APU drive shaft at the second rotational speed falling within a second speed range which ranges from about 22,001 RPM to about 24,034 RPM.