Editing Cabin pressurization (section)

==Mechanics==
[[File:High performance airplane pressurization system.png|thumb|right|Piston-engine aircraft cabin pressurization using a dedicated compressor.<ref>{{Cite PHAK|year=2016|chapter=7|pages=34—35}}</ref>]]

An airtight fuselage is pressurized using a source of compressed air and controlled by an [[environmental control system]] (ECS). The most common source of compressed air for pressurization is [[bleed air]] from the compressor stage of a [[gas turbine]] engine; from a low or intermediate stage or an additional high stage, the exact stage depending on engine type. By the time the cold outside air has reached the bleed air valves, it has been heated to around {{cvt|200|°C|lk=on}}. The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight. Piston-engine aircraft require an additional compressor, see diagram right.<ref name="Engineering Aspects of Cabin Air">{{cite web |url=http://www.cabinfiles.com/?CFrequest=file;03032001100119 |title=Commercial Airliner Environmental Control System: Engineering Aspects of Cabin Air |url-status=dead |format=PDF |year=1995 |archive-url=https://web.archive.org/web/20120331055732/http://www.cabinfiles.com/?CFrequest=file%3B03032001100119 |archive-date=31 March 2012 }}</ref>

The part of the bleed air that is directed to the ECS is then expanded to bring it to cabin pressure, which cools it. A final, suitable temperature is then achieved by adding back heat from the hot compressed air via a [[heat exchanger]] and [[air cycle machine]] known as a PAC (Pressurization and Air Conditioning) system. In some larger airliners, hot trim air can be added downstream of air-conditioned air coming from the packs if it is needed to warm a section of the cabin that is colder than others.

[[File:Outflow Valve and Pressure Relief Valve on a Boeing 737-800.jpg|thumb|left|Outflow and pressure relief valve on a [[Boeing 737 Next Generation|Boeing 737-800]]]]
At least two engines provide compressed bleed air for all the plane's pneumatic systems, to provide full [[Redundancy (engineering)|redundancy]]. Compressed air is also obtained from the [[auxiliary power unit]] (APU), if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started. Most modern commercial aircraft today have fully redundant, duplicated electronic controllers for maintaining pressurization along with a manual back-up control system.

All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief valve, in addition to other safety relief valves. If the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position so that the cabin altitude is as low as practical without exceeding the maximum pressure differential limit on the fuselage. The pressure differential varies between aircraft types, typical values are between {{cvt|7.8|psi|hPa|order=flip|lk=on}} and {{cvt|9.4|psi|hPa|order=flip|lk=on}}.<ref name="Differential Pressure Characteristics of Aircraft">{{cite web |url=http://mrcabinpressure.com/aircraft.htm|title=Differential Pressure Characteristics of Aircraft}}</ref> At {{cvt|39000|ft|0}}, the cabin pressure would be automatically maintained at about {{cvt|6900|ft}}, ({{cvt|450|ft}} lower than Mexico City), which is about {{cvt|11.5|psi|hPa|order=flip}} of atmosphere pressure.<ref name="Engineering Aspects of Cabin Air"/>

Some aircraft, such as the [[Boeing 787 Dreamliner]], have re-introduced electric compressors previously used on piston-engined airliners to provide pressurization.<ref name=Design_News_20070604>{{cite news |url=http://www.designnews.com/document.asp?doc_id=222308 |title=Boeing's 'More Electric' 787 Dreamliner Spurs Engine Evolution: On the 787, Boeing eliminated bleed air and relied heavily on electric starter generators |work=[[Design News]] |date=June 4, 2007 |editor-last=Ogando |editor-first=Joseph |access-date=September 9, 2011 |archive-date=April 6, 2012 |archive-url=https://web.archive.org/web/20120406062451/http://www.designnews.com/document.asp?doc_id=222308 |url-status=dead }}</ref><ref>{{cite web |url=http://aviationweek.com/awin/massive-787-electrical-system-pressurizes-cabin |title=Massive 787 Electrical System Pressurizes Cabin |last=Dornheim |first=Michael |work=Aviation Week & Space Technology |date=March 27, 2005 |url-access=subscription }}</ref> The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer;<ref name="AERO_QTR_406_787_from_the_ground_up">[http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_06/AERO_Q406_article4.pdf "Boeing 787 from the Ground Up"]</ref> therefore, it is unclear whether this increases the overall efficiency of the aircraft air handling system. They do, however, remove the danger of [[Fume event|chemical contamination of the cabin]], simplify engine design, avert the need to run high pressure pipework around the aircraft, and provide greater design flexibility.