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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." 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." 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).
In civil aircraft, bleed air's primary use is to provide pressure for the aircraft cabin by supplying air to the environmental control system. Additionally, bleed air is used to keep critical parts of the aircraft (such as the wing leading edges) ice-free.
Bleed air is used on many aircraft systems because it is easily available, reliable, and a potent source of power. For example, bleed air from an airplane engine is used to start the remaining engines. Lavatory water storage tanks are pressurized by bleed air that is fed through a pressure regulator.
When used for cabin pressurization, the bleed air from the engine must first be cooled (as it exits the compressor stage at temperatures as high as 250 °C) by passing it through an air-to-air heat exchanger cooled by cold outside air. It is then fed to an air cycle machine unit which regulates the temperature and flow of air into the cabin, keeping the environment comfortable.
Bleed air is also used to heat the engine intakes. This prevents ice from accumulating, breaking loose, and being ingested by the engine, possibly damaging it.
On aircraft powered by jet engines, a similar system is used for wing anti-icing by the 'hot-wing' method. In icing conditions, water droplets condensing on a wing's leading edge can freeze. If that happens, the ice build-up adds weight and changes the shape of the wing, causing a degradation in performance, and possibly a critical loss of control or lift. To prevent this, hot bleed air is pumped through the inside of the wing's leading edge, heating it to a temperarture above freezing, which prevents the formation of ice. The air then exits through small holes in the wing edge.
Bleed air from the high-pressure compressor of the engine is used to supply reaction control valves as used for part of the flight control system in the Harrier jump jet family of military aircraft.
On rare occasions, bleed air used for air conditioning and pressurization can be contaminated by chemicals such as oil or hydraulic fluid. This is known as a fume event. While those chemicals can be irritating, such rare events have not been established to cause long term harm.
Certain neurological and respiratory ill health effects have been linked anecdotally to exposure to contaminated bleed air on commercial and military aircraft. This alleged long-term illness is referred to as aerotoxic syndrome by agenda groups, but it is not a medically recognized syndrome. One potential contaminant is tricresyl phosphate. Cabin Environment Research is one of many functions of the ACER Group, but no causal relationship has been established yet by researchers.
A number of lobbying groups have been set up to advocate for research into this hazard. These groups include the Aviation Organophosphate Information Site (AOPIS) (2001), the Global Cabin Air Quality Executive (2006) and the UK-based Aerotoxic Association (2007). In March 2010, in a court in Australia, a former flight attendant won a case against her former employer for chronic respiratory problems after she was exposed to oil fumes on a flight in March 1992.
A study made for the EU in 2014 confirmed that contamination is a problem. 
Bleed air systems have been in use for several decades in passenger jets. Recent improvements in solid state electronics have enabled pneumatic power systems to be replaced by electric power systems. In a bleedless aircraft such as the Boeing 787, each engine has 2 variable frequency electrical generators to compensate for not providing compressed air to external systems. Eliminating bleed air and replacing it with extra electric generation is believed to provide a net improvement to engine efficiency, lower weight and easier maintenance.
A bleedless aircraft achieves fuel efficiency by eliminating the process of compressing and decompressing air, and by reducing the aircraft's mass due to the removal of ducts, valves, heat exchangers, and other heavy equipment.
The APU (auxiliary power unit) does not need to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. By driving cabin air supply compressors at the minimum required speed, no energy wasting modulating valves are required. High temperature, high-pressure air cycle machine (ACM) packs can be replaced with low temperature, low pressure packs to increase efficiency. At cruise altitude, where most aircraft spend the majority of their time and burn the majority of their fuel, the ACM packs can be bypassed entirely, saving even more energy. Since no bleed air is taken from the engines for the cabin, the potential of engine oil contamination of the cabin air supply is eliminated.
Lastly, advocates of the design say it improves safety as heated air is confined to the engine pod, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak could damage surrounding systems.
In the 787, cabin air enters from under the fuselage and is compressed as required. De-icing is achieved by electric thermal blankets in the wing. Hydraulic pumps for flaps, slats, speed brakes and other control surfaces are also powered electrically.
Eliminating bleed air increases the electric load, as cabin pressurization, anti-ice/de-ice systems, and other functions need to be powered electrically instead. This necessitates an increased size of electrical generators as well as higher wattage power distribution boards and more sophisticated back up and control systems.