Bleed air (or customer bleed air) in gas turbine engines is compressed air that can taken from within the engine, most often after the compressor stage(s) but before the fuel is injected in the burners. While bleed air could be drawn in any gas turbine engine, its usage is generally restricted to 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.
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. Even the outside air probe on some aircraft utilize bleed air to drive a venturi pump to draw outside air into a temperature sensor chamber. Early jet aircraft used bleed air to drive the gyroscopes in their cockpit artificial horizons.
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 300 °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. A small amount of bleed air is taken from the engine and piped to the engine pod shroud, where it heats the back side of the fan case. This prevents ice from accumulating, breaking loose, and being ingested by the engine, possibly damaging it.
A similar system is used for wing de-icing by the 'hot-wing' method. In icing conditions, water droplets condensing on a wing's leading edge can freeze. This build-up of ice 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, warm bleed air is pumped through the inside of the wing's leading edge. This heats up the metal and prevents the formation of ice. The air then exits through small holes in the wing edge. Alternatively, bleed air may be used to inflate a rubber boot on the leading edge, breaking the ice loose.
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.
Bleed air systems have been in use for several decades in passenger jets. In a bleedless aircraft such as the 787, the engines operate without providing bleed air. Eliminating bleed air improves engine efficiency, as there is no loss of mass airflow and therefore energy from the engine, leading to lower fuel consumption.
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.
Eliminating bleed air increases load on the aircraft generators for cabin pressurization, anti-ice/de-ice systems, and other functions previously covered by bleed air. This may necessitate an increased size of such generators.
Occasionally, bleed air used for air conditioning and pressurization is contaminated by chemicals such as oil or hydraulic fluid. This is known as a fume event. Chemicals such as these are irritating, but 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. One potential contaminant is tricresyl phosphate. Academic research is ongoing, such as that by University of New South Wales, Cranfield University, University of British Columbia, University of Washington, University College London (Sarah Mackenzie Ross), and the ACER Group (Airliner Cabin Environment Research), 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 potential 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.
- Cabin pressurization
- Environmental control system (aircraft)
- Aerotoxic syndrome
- Ice protection system
- Discussion paper on the cabin air environment, COT, 2006
- "Bleed Air Systems". Skybrary.aero. Retrieved January 1, 2013.
- "Ice Protection Systems". Skybrary. Retrieved January 1, 2013.
- "Technical" page on harrier.org.uk website, viewed 2013-11-24
- AERO 787 No Bleed Systems The Boeing Company 2008
- Sinnett, Mike (2008). "787 No-Bleed Systems". Boeing. Retrieved January 1, 2013.
- "UK Comittee on Toxicology Leaflet". Retrieved December 31, 2012.
- Nassauer, Sarah (July 30, 2009). "Up in the Air: New Worries About 'Fume Events' on Planes". Wall Street Journal. Retrieved December 31, 2012.
- "Skydrol FAQ". Skydrol. Retrieved December 31, 2012.
- Ill health Following Exposure to Contaminated Aircraft Air: Psychosomatic Disorder or Neurological Injury?, Dr Sarah Mackenzie Ross, 2006
- "Airliner Cabin Environment Research".
- Bagshaw, Michael (September 2008). "The Aerotoxic Syndrome". European Society of Aerospace Medicine. Retrieved December 31, 2012.
- Select Committee on Science and Technology (2000). Science and Technology – Fifth Report (Report). House of Lords. http://www.publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12107.htm. Retrieved 2010-07-05.
- Turner v Eastwest Airlines Limited (2009) at Dust Diseases Tribunal of New South Wales