Bleed air in gas turbine engines is compressed air that is taken from the compressor stage of the engine, which is upstream of the fuel burning section. In modern airliner engines, two regulator valves (Hi stage and Low stage) turn on and off automatically and are controlled by at least “…two air supply and cabin pressure controllers (ASCPCs) which open and close appropriate valves. Engine Bleed Air comes from the high stage or low stage engine compressor section. Low stage air is used during high power setting operation and high stage air is used during descent and other low power setting operations.”^[1][2] Bleed air from that system can be used for internal cooling of the engine, cross-starting another engine, engine and airframe anti-icing, cabin pressurization, pneumatic actuators, air-driven motors, pressurizing the hydraulic reservoir, waste and water storage tanks. Some engine maintenance manuals refer to such systems as “Customer Bleed Air.”^[3][4][5] Bleed air is valuable in an aircraft for two properties: high temperature and high pressure (typical values are 200–250 °C and 275 kPa (40 PSI), for regulated bleed air exiting the engine pylon for use throughout the aircraft).

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