What does the term "bleed air" refer to in relation to the APU?

The term "bleed air" refers specifically to extracted compressed air that is taken from the APU's compressor section. This pressurized air is primarily utilized for starting the engines of an aircraft. When the APU is operational, it generates enough compressed air to provide the necessary pressure for engine start, as well as for other pneumatic systems on the aircraft. This function is critical for ensuring that the engines can be started efficiently while on the ground without needing ground support equipment.

This understanding of bleed air is vital for aircraft operation, as it plays a crucial role in various systems beyond just engine starting, including cabin pressurization, de-icing, and air conditioning when the engines are not running. In the context of the C-5 APU, recognizing the importance and application of bleed air helps in assessing its function and troubleshooting any issues that may arise related to engine start or other pneumatic functions.