Bleed air

Bleed air is compressed air taken from the compressor stage of a gas turbine upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPCs) valves bleed air from high or low stage engine compressor sections. Low stage air is used during high power setting operation, and high during descent and other low power setting operations. Bleed air from that system can be utilized 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, and 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). Bleed air is compressed air taken from the compressor stage of a gas turbine upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPCs) valves bleed air from high or low stage engine compressor sections. Low stage air is used during high power setting operation, and high during descent and other low power setting operations. Bleed air from that system can be utilized 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, and 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 plane (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 that 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 forming, 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 temperature above freezing, which prevents the formation of ice. The air then exits through small holes in the wing edge. On propeller-driven aircraft, it is common to use bleed air to inflate a rubber boot on the leading edge, breaking the ice loose after it has already formed. 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.