What is the function of an aircraft's APU?

The function of an aircraft's Auxiliary Power Unit (APU) is primarily to provide power and bleed air when the engines are not running. The APU is a small turbine engine located at the rear of the aircraft that operates independently from the main engines. Its main roles include supplying electrical power to start the aircraft's engines and providing bleed air for air conditioning, pressurization, and engine starting.

When the aircraft is on the ground and the engines are off, the APU ensures that essential systems continue to operate without relying on the main engines. This is important for passenger comfort, maintaining cabin pressure, and ensuring that critical electronic systems remain functional while the aircraft is in a stationary state.

The other options do not accurately reflect the primary roles of the APU. For instance, while generating electricity is a function of the APU, it is more comprehensive to say that its primary purpose is to provide both power and bleed air, not just electricity alone. Similarly, the APU does not provide thrust for takeoff or assist in navigation and communication, as these functions are specifically related to the aircraft's engines and avionics systems.