What is the primary purpose of the APU?

The primary purpose of the Auxiliary Power Unit (APU) is to provide 115 volts AC electrical power. The APU is a small engine located at the rear of the aircraft that generates power while the aircraft is on the ground or during flight when the main engines are not running. This power is essential for starting the main engines, running the air conditioning and other electrical systems, and providing power to auxiliary services without relying on the main engines.

When the aircraft is on the ground, the APU can be used to supply electrical power to the aircraft systems and cabin, ensuring that important systems remain operational and providing a comfortable environment for passengers and crew. It also enables maintenance checks and equipment operation when the main engines are off.

While the APU can provide hydraulic power, assist in engine operation, and contribute to passenger comfort indirectly, those are not its primary uses. The provision of electrical power is its key function, making it crucial for overall aircraft operations, especially when the main engines are not running.