Introduction
Sources of Bleed Air
Bleed air is high-pressure, high-temperature air extracted from one or more sources on the aircraft and distributed throughout various pneumatic systems. It plays a critical role in supporting essential functions such as engine starting, cabin pressurization, air conditioning, anti-icing, and hydraulic reservoir pressurization. While the primary source is typically the main engines, modern aircraft may also draw bleed air from auxiliary systems including the Auxiliary Power Unit (APU), external Ground Power Units (GPUs), or dedicated Air Starter Units (ASUs). Understanding the origins and roles of bleed air is fundamental to grasping how key aircraft systems operate—particularly during ground operations, takeoff, and cruise flight.
Systems That Rely on Bleed Air
Many aircraft systems rely on bleed air, extracted from the compressor stages of a gas turbine engine. Alternative sources include engine-driven compressors, blowers, APU bleed air, and ground power units. Since system designs vary between aircraft, detailed knowledge is unnecessary.
