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Bleed air is compressed air taken from aircraft turbine engines for cabin climate control and systems such as de-icing equipment. Passengers traveling at altitudes above approximately 12,000 feet (3,600 meters) may need auxiliary breathing air to prevent hypoxia, or oxygen starvation. Commercial aircraft use bleed systems to pressurize the entire cabin rather than supplying individual passengers.
An aircraft cabin is a closed environment at high altitudes, because the atmosphere outside contains too little oxygen to sustain life. Pressure valves control the cabin pressure at a level approximately four times the outside pressure, so an aircraft traveling at 36,000 feet (11,000 meters) will have a cabin pressure equivalent to roughly 8000 feet (2,400 meters). Bleed air is needed to control cabin temperature and pressurize the cabin for life support.
Turbine engines take thin high-altitude air and compress it through a series of rotary vanes. The compressed air is mixed with fuel and ignited, creating thrust that moves the aircraft forward. Bleed air is taken from the compressed air supply before the fuel mixing, and is sent to a series of coolers and filters prior to being sent to the passenger cabin. It may also be used for some systems that require an air pressure source rather than electrical power.
Even though the aircraft may be flying in very cold temperatures at high altitudes, the compressed air can be very hot. The coolers reduce the bleed air temperature either by running it through heat exchangers exposed to the cold air, or by reducing its pressure, or flashing, through a control valve. Normally combinations of cooling methods are used to provide the proper temperatures and pressures.
Cabin pressurization uses some of the engine's horsepower, so more fuel is burned per flight for bleed systems. Beginning in the 21st century, aviation designers developed no-bleed systems, using electrical power for systems, and incorporating compressors rather than using engine air. One commercial aircraft company estimated that these changes could reduce fuel use by three percent. Additional savings occur because much of the equipment needed for cooling and pressure control of cabin air and other systems can be eliminated.
A concern of closed aircraft cabins is removing pollutants before they can reach dangerous levels. An issue that became more visible in the later 20th century was health hazards from pollutants introduced by the bleed air systems. This became known as aerotoxic syndrome, and flight crews and frequent passengers complained of respiratory and other problems. The high engine temperatures might decompose hydraulic fluids, fuels and lubricants in the engine compartments, with the resulting gases sent to the cabin through the air system. This issue was another driving force for developing no-bleed cabin air systems.