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 (400–500 °F) 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 (500 °F) 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 family of military aircraft.
On about 1 in 5,000 flights,  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 events have not been established to cause long-term harm.  
Certain neurological and respiratory ill health effects have been linked 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, but it is not a medically recognized syndrome. One potential contaminant is tricresyl phosphate. 
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,  but their researchers have not yet established any causal relationship.  
Although a study made for the EU in 2014 confirmed that contamination of cabin air could be a problem, that study also stated:
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.  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. 
The FAA has revoked the medical certificates of several pilots who developed neurological issues after fume events.  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. 
In July 2015, pilots on a Spirit Airlines flight were partially incapacitated by fumes in bleed air. 
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, 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. 
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.” 
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. 
An airliner is a type of aircraft for transporting passengers and air cargo. Such aircraft are most often operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an airplane intended for carrying multiple passengers or cargo in commercial service. The largest of them are wide-body jets which are also called twin-aisle because they generally have two separate aisles running from the front to the back of the passenger cabin. These are usually used for long-haul flights between airline hubs and major cities. A smaller, more common class of airliners is the narrow-body or single-aisle. These are generally used for short to medium-distance flights with fewer passengers than their wide-body counterparts.
The Kegworth air disaster occurred when British Midland Airways Flight 092, a Boeing 737-400, crashed onto the motorway embankment between the M1 motorway and A453 road near Kegworth, Leicestershire, England, while attempting to make an emergency landing at East Midlands Airport on 8 January 1989.
The Pratt & Whitney JT8D is a low-bypass turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered the US Navy A-6 Intruder and A-4 Skyhawk attack aircraft. Eight models comprise the JT8D standard engine family, covering the thrust range from 12,250 to 17,400 pounds-force, and power 727, 737-100/200, and DC-9. The updated JT8D-200 family, covering the 18,900 to 21,000 pounds-force, powers the MD-80 and re-engined Super 27 aircraft. The JT8D was built under license in Sweden as the Volvo RM8, a redesigned afterburning derivative for the Saab 37 Viggen fighter. Pratt & Whitney also sells static versions for powerplant and ship propulsion as the FT8.
The General Electric GEnx is an advanced dual rotor, axial flow, high-bypass turbofan jet engine in production by GE Aviation for the Boeing 787 and 747-8. The GEnx is intended to succeed the CF6 in GE's product line.
An uncontrolled decompression is an undesired drop in the pressure of a sealed system, such as an aircraft cabin or hyperbaric chamber, that typically results from human error, structural failure, or impact, causing the pressurised vessel to vent into its surroundings or fail to pressurize at all.
In aeronautics, an environmental control system (ECS) of an aircraft is an essential component which provides air supply, thermal control and cabin pressurization for the crew and passengers. Additional functions include the cooling of avionics, smoke detection, and fire suppression.
Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for passengers and crew flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic, tanks. The air is cooled, humidified, and mixed with recirculated air if necessary before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.
An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an air conditioning pack. The air cycle cooling process uses air instead of a phase changing material such as Freon in the gas cycle. No condensation or evaporation of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.
The Learjet 25 is an American ten-seat, twin-engine, high-speed business jet aircraft manufactured by Learjet. It is a stretched version of the Learjet 24.
In aeronautics, ice protection systems keep atmospheric moisture from accumulating on aircraft surfaces, such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. Ice buildup can change the shape of airfoils and flight control surfaces, degrading control and handling characteristics as well as performance. An anti-icing, de-icing, or ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it becomes dangerous.
Aircraft emergency oxygen systems or air masks are emergency equipment fitted to pressurized commercial aircraft, intended for use when the cabin pressurisation system has failed and the cabin altitude has climbed above a safe level. It consists of a number of individual yellow oxygen masks stored in compartments near passenger seats and near areas like lavatories and galleys, and an oxygen source, like a centralized gaseous cylinder or decentralized chemical oxygen generator.
A turbine engine failure occurs when a turbine engine unexpectedly stops producing power due to a malfunction other than fuel exhaustion. It often applies for aircraft, but other turbine engines can fail, like ground-based turbines used in power plants or combined diesel and gas vessels and vehicles.
Welcome Aboard Toxic Airlines is a 2007 British documentary film about aerotoxic syndrome directed and produced by former airline captain Tristan Loraine.
This article briefly describes the components and systems found in jet engines.
The Aerotoxic Association was founded on 18 June 2007, at the British Houses of Parliament by former BAe 146 Training Captain John Hoyte, to raise public awareness about the ill health allegedly caused after exposure to airliner cabin air that he claimed been contaminated to toxic levels, by engine oil leaking into the bleed air system, which pressurizes all jet aircraft, with the exception of the Boeing 787.
Aerotoxic syndrome relates to ill-health effects that are claimed to be caused by breathing contaminated airliner cabin air. This condition is not an established medical diagnosis.
A fume event occurs when bleed air used for cabin pressurization and air conditioning in a pressurized aircraft is contaminated by fluids such as engine oil, hydraulic fluid, anti-icing fluid, and other potentially hazardous chemicals.
A Dark Reflection is a 2015 British independent investigative thriller film directed and produced by former British Airways airline captain Tristan Loraine. Billed as Erin Brockovich meets All the President's Men and as a fact-based investigative thriller, the film is based on the director's own experience as a commercial pilot.
Nathan C. Price was an American engineer and inventor. He made substantial contributions to several US aircraft projects during the first half of the twentieth century.
Susan Michaelis is a former Australian flight instructor and airline transport (ATPL) pilot. She is also a researcher working on the issue of contaminated air on aircraft and the health effects of exposure to heated jet engine oils and hydraulic fluids known to contaminate the breathing air supply on aircraft, often called Aerotoxic Syndrome. She is a part of the University of Stirling’s Occupational and Environmental Health Research group where she is a Honorary Sensor Research Fellow.