Editing Bleed air

{{Short description|Aircraft gas turbine function}}

'''Bleed air''' in [[aerospace engineering]] 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 (ASCPC) valves bleed air from low or high stage engine compressor sections; low stage air is used during high power setting operation, and high stage air is used during descent and other low power setting operations.<ref name="777 Bleed Air">{{cite web |title=777 Bleed Air |url=http://meriweather.com/flightdeck/777/over/bleed.html |url-status=dead |access-date=2014-02-23 |archive-date=2014-11-13 |archive-url=https://web.archive.org/web/20141113080737/http://www.meriweather.com/flightdeck/777/over/bleed.html }}</ref><ref name="Global 300 Bleed Air">{{cite web | url=http://www.smartcockpit.com/download.php?path=docs/&file=Bombardier_Challenger_Global_300-Air_Cond_and_Press.pdf | title=Global 300 Bleed Air | access-date=2019-06-11 | archive-url=https://web.archive.org/web/20160327092756/http://www.smartcockpit.com/download.php?file=bombardier_challenger_global_300-air_cond_and_press.pdf&path=docs%2F | archive-date=2016-03-27 | url-status=dead }}</ref> 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 actuator]]s, 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".<ref name="US Navy">{{cite web | url=https://www.fas.org/man/dod-101/navy/docs/swos/eng/64b7-205.html | title=Naval Operations Manual}}</ref><ref name="European Space Agency">{{cite web | url=http://www.easa.europa.eu/certification/type-certificates/docs/engines/EASA-TCDS-E.021_%28IM%29_GE_CF34--10E_series_engines-05-03012013.pdf | title=European Space Agency}}</ref><ref name="mil-specs">{{cite web | url=http://mil-spec.tpub.com/MIL-E/MIL-E-8593A/MIL-E-8593A00103.htm | title=mil-spec}}</ref> 

Bleed air is valuable in an aircraft for two properties: high [[temperature]] and high [[pressure]] (typical values are {{cvt|200–250|°C|sigfig=1}} and {{cvt|275|kPa|sigfig=1}}, for regulated bleed air exiting the engine pylon for use throughout the aircraft).

==Uses==
[[File:Cabin pressure and Bleed air control panels on a Boeing 737-800.jpg|thumb|right|upright|Cabin pressure and bleed air controls in a [[Boeing 737-800]]]]
[[File:ECSB737-300ENGINES ON.svg|thumb|right|Environmental control system (ECS) schematic of [[Boeing 737-300]]]]

In civil aircraft, bleed air's primary use is to provide pressure for the aircraft [[cabin (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 edge]]s) ice-free.<ref name="skybrary">{{cite web | url=http://www.skybrary.aero/index.php/Bleed_Air_Systems | title=Bleed Air Systems | publisher=Skybrary.aero | access-date=January 1, 2013}}</ref>

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. [[Aircraft lavatory|Lavatory]] water storage tanks are pressurized by bleed air that is fed through a [[pressure regulator]].<ref name="skybrary" />

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 {{cvt|250|°C|sigfig=1}} by passing it through an air-to-air [[heat exchanger]] cooled by the 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.<ref name="skybrary" /> This process is contrary to the [[List of common misconceptions|common misconception]] that cabin air in airplanes is the same air being recycled.<ref>{{Cite web |last=Villazon |first=Luis |title=How 'stale' is the recycled air in a plane? |url=https://www.sciencefocus.com/science/how-stale-is-the-recycled-air-in-a-plane |access-date=July 6, 2024 |website=[[BBC Science Focus]]}}</ref>

Bleed air is also used to heat the engine [[intake]]s. This prevents ice from forming, accumulating, breaking loose, and being ingested by the engine, which could damage it.<ref name="skybrary2">{{cite web | url=http://www.skybrary.aero/index.php/Ice_Protection_Systems | title=Ice Protection Systems | publisher=Skybrary | access-date=January 1, 2013}}</ref>

On aircraft powered by jet engines, a similar system is used for [[Ice protection system|wing anti-icing]] by the 'hot-wing' method. In icing conditions, water droplets [[condensation|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 (force)|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.<ref name="skybrary" /><ref name="skybrary2" />

Bleed air from the high-pressure compressor of the engine is used to supply [[Harrier jump jet#Specifications|reaction control valves]] as used for part of the [[flight control system]] in the [[Harrier jump jet|Harrier]] family of military aircraft.

==Contamination==
{{Main|Fume event}}

On about 1 in 5,000 flights,<ref name="la-2020-12-172">{{Cite web|last=Feldman|first=Kiera|title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes|url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/|website=Los Angeles Times|date=17 December 2020 }}</ref> bleed air used for air conditioning and pressurization can be contaminated by chemicals such as oil or hydraulic fluid.<ref name=WSJ_>{{cite web | url=https://www.wsj.com/articles/SB10001424052970204900904574302293012711628 | title=Up in the Air: New Worries About 'Fume Events' on Planes | publisher=Wall Street Journal | date=30 July 2009 | access-date=29 December 2012 | author=Sarah Nassauer}}</ref> This is known as a fume event. While those chemicals can be irritating, such events have not been established to cause long-term harm.<ref name="WSJ">{{cite web | url=https://www.wsj.com/articles/SB10001424052970204900904574302293012711628 | title=Up in the Air: New Worries About 'Fume Events' on Planes | publisher=Wall Street Journal | date=July 30, 2009 | access-date=December 31, 2012 | author=Nassauer, Sarah}}</ref><ref>{{cite web | url=http://www.skydrol.com/pages/faqs.asp | title=Skydrol FAQ | publisher=Skydrol | access-date=December 31, 2012}}</ref>

Certain neurological and respiratory ill health effects have been linked [[anecdotal evidence|anecdotally]] to exposure to bleed air that has been alleged to have been contaminated with toxic levels on commercial and military aircraft. This alleged long-term illness is referred to as [[Aerotoxic Syndrome|aerotoxic syndrome]], but it is not a medically recognized syndrome. One potential contaminant is [[tricresyl phosphate]].<ref name="la-2020-12-17">{{Cite web|last=Feldman|first=Kiera|title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes|url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/|website=Los Angeles Times|date=17 December 2020 }}</ref>

Many lobbying groups have been set up to advocate for research into this hazard, including the Aviation Organophosphate Information Site (AOPIS) (2001), the Global Cabin Air Quality Executive (2006) and the UK-based [[Aerotoxic Association]] (2007). Cabin Environment Research is one of many functions of the ACER Group,<ref>{{cite web|url= http://www.acer-coe.org/index.html|title= Airliner Cabin Environment Research|access-date= 2013-07-16|archive-url= https://web.archive.org/web/20130728040148/http://acer-coe.org/index.html|archive-date= 2013-07-28|url-status= dead}}</ref> but their researchers have not yet established any [[Causality|causal relationship]].<ref>{{cite web | url=http://www.esam.aero/main/docs/ecam08/No%207%20Bagshaw%20paper.pdf | title=The Aerotoxic Syndrome | publisher=European Society of Aerospace Medicine | date=September 2008 | access-date=December 31, 2012 | author=Bagshaw, Michael | url-status=dead | archive-url=https://web.archive.org/web/20100827071537/http://www.esam.aero/main/docs/ecam08/No%207%20Bagshaw%20paper.pdf | archive-date=August 27, 2010 }}</ref><ref>{{cite report|chapter-url=https://publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12107.htm|author=Select Committee on Science and Technology|publisher=House of Lords|title=Science and Technology – Fifth Report|chapter=Chapter 4: Elements Of Healthy Cabin Air|access-date=2010-07-05|year=2000}}</ref>

Although a study made for the EU in 2014 confirmed that contamination of cabin air could be a problem, that study also stated:

::"A lot of reported fume events caused comfort limitations for the occupants but posed no danger. A verification of cabin air contamination with toxic substances (e.g. TCP/TOCP) was not possible with the fume events the BFU investigated."<ref name="EU Study">{{cite web | url=http://www.bfu-web.de/EN/Publications/Safety%20Study/Studies/140507_Fume_Events.pdf?__blob=publicationFile | title=Study of Reported Occurrences in Conjunction with Cabin Air Quality in Transport Aircraft | publisher=German Federal Bureau of Aircraft Accident Investigation|year=2014}}</ref>

While no scientific evidence to date has found that airliner cabin air has been contaminated to toxic levels (exceeding known safe levels, in ppm, of any dangerous chemical), a court in Australia in March 2010 found in favor of a former airline flight attendant who claimed she suffered chronic respiratory problems after being exposed to oil fumes on a trip in March 1992.<ref>[http://www.austlii.edu.au/au/cases/nsw/NSWDDT/2009/10.html Turner v Eastwest Airlines Limited (2009) at Dust Diseases Tribunal of New South Wales]</ref> Such testing is infrequent due to Boeing's refusal to install air quality sensors in its planes, fearing lawsuits from crew or passengers over fume events, and airlines refused to allow flight attendants to carry air samplers after Congress mandated chemical measurements.<ref name="la-2020-12-174">{{Cite web|last=Feldman|first=Kiera|title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes|url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/|website=Los Angeles Times|date=17 December 2020 }}</ref>

The FAA has revoked the medical certificates of several pilots who developed neurological issues after fume events.<ref name="la-2020-12-173">{{Cite web|last=Feldman|first=Kiera|title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes|url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/|website=Los Angeles Times|date=17 December 2020 }}</ref> A judge who awarded workers' compensation to a pilot who had suffered [[toxic encephalopathy]] (brain damage) from a fume event condemned the airline industry's obstructionism around fume events.<ref name="la-2020-12-174" />

In July 2015, pilots on a [[Spirit Airlines]] flight were partially incapacitated by fumes in bleed air.<ref>{{cite web |last1=Feldman |first1=Kiera |title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes |url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/ |website=Los Angeles Times |date=17 December 2020 |language=en}}</ref>

==Bleedless aircraft==
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 Dreamliner|Boeing 787]], each engine has two 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 in engine efficiency, lower weight, and ease of maintenance.<ref>[http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/AERO_Q407_article2.pdf AERO 787 No Bleed Systems] The Boeing Company 2008</ref>

According to Boeing internal documents, eliminating the use of bleed air as a source of cabin air also translates into the “elimination of engine contaminants potentially entering cabin air supply.”<ref>{{Cite web|last=Feldman|first=Kiera|title='We are slowly being poisoned.' How toxic fumes seep into the air you breathe on planes|url=https://www.latimes.com/projects/toxic-chemicals-planes-covid-19-travel-woes/|website=Los Angeles Times|date=17 December 2020 }}</ref>

===Benefits===
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.<ref name="boeing">{{cite web | url=http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_1.html | title=787 No-Bleed Systems | publisher=Boeing | year=2008 | access-date=January 1, 2013 | author=Sinnett, Mike}}</ref>

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.<ref name="boeing" />

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.<ref name="boeing" />

==See also==
* [[Aerotoxic syndrome]]
* [[Cabin pressurization]]
* [[Components of jet engines]]
* [[Environmental control system (aircraft)]]
* [[Ice protection system]]

==References==
{{Reflist}}

{{Aircraft gas turbine engine components}}
{{Aircraft components}}

{{DEFAULTSORT:Bleed Air}}
[[Category:Aerospace engineering]]
[[Category:Jet engines]]
[[Category:Occupational safety and health]]
[[Category:Aircraft systems|Bleed air]]