Which of the following is NOT a main component of a typical APU?

A typical Auxiliary Power Unit (APU) is designed primarily to provide electrical power and bleed air to assist in starting the main engines and to power onboard systems while on the ground. The main components of an APU generally include a small power turbine that generates both electrical power and bleed air for air conditioning or engine start. The electrical generator is crucial for supplying power to the aircraft's electrical systems when the engines are not running. Additionally, a fuel system is essential as it supplies the APU with fuel from the aircraft's tanks for combustion and operation.

In contrast, while hydraulic systems are vital in aircraft operation for controlling flight surfaces, landing gears, and brakes, they are not a direct component of the APU itself. The APU does not typically contribute hydraulic power; that function is more commonly associated with the main engines or dedicated hydraulic systems in the aircraft. Therefore, the hydraulic pump is not considered a main component of a typical APU.