Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft. 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). This compressed air can be used within the aircraft in many different ways, from de-icing, to pressurizing the cabin, to pneumatic actuators. However, bleed air is quite hot and when being used in the cabin or other low temperature areas, it must first be cooled or even refrigerated by the aircraft's environmental control system (ECS). Newer aircraft rely more on electricity, reducing the need for compressed air. Since most gas turbine engines use multiple compressor stages, some newer engines have the bleed air inlet between compressor stages to reduce the temperature of the compressed air.
Read more about Bleed Air: Merits of Bleed Air, Recent Developments in Civil Aircraft, Controversy
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