Bleed air
Encyclopedia
Bleed air in gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. Bleed air is valuable in an aircraft for two properties: high temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and high pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 (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. However, bleed air is quite hot and when being used in the cabin or other low temperature areas, it must first be cooled or even refrigerated by the aircraft's environmental control system
Environmental Control System
The environmental control system of an aircraft provides air supply, thermal control and cabin pressurization for the crew and passengers...

 (ECS). Newer aircraft rely more on electricity, reducing the need for compressed air. Since most gas turbine engines use multiple compressor stages, some newer engines have the bleed air inlet between compressor stages to reduce the temperature of the compressed air.

Merits of bleed air


In civil aircraft, bleed air's primary use is to provide pressure for the aircraft cabin by supplying air to the Environmental Control System
Environmental Control System
The environmental control system of an aircraft provides air supply, thermal control and cabin pressurization for the crew and passengers...

. Additionally, bleed air is used to keep critical parts of the aircraft (such as the wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

 leading edge
Leading edge
The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one....

s) ice-free.

Bleed air is used on many aircraft systems because it is easily available, reliable, and a potent source of power. For example, air turbine starters used to start large jet engines are much smaller and lighter than an electric motor of equivalent power output. Bleed air for starting is provided by an on board Auxiliary Power Unit
Auxiliary power unit
An auxiliary power unit is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.-Function:...

 (APU) or an external huffer
Huffer
In aviation, a huffer is an external engine that creates large volumes of pressurized air used to start large gas turbine jet engines on some types of aircraft....

 if the APU is inoperative. Once a single engine is started, its bleed air can be used to drive the starter on the remaining engines. Lavatory
Aircraft lavatory
An aircraft lavatory is a small room on an aircraft with a toilet and sink.-Minimum standards:Lavatories per passenger provided aboard aircraft vary considerably from airline to airline and aircraft to aircraft...

 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 in to a temperature sensor chamber. Early jet aircraft even used bleed air to drive the gyroscopes in their cockpit artificial horizons
Attitude indicator
An attitude indicator , also known as gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch and bank or roll and is a primary instrument for flight in instrument meteorological conditions...

.

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
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 cooled by cold outside air. It is then fed to an air cycle machine
Air cycle machine
An air cycle machine is the refrigeration unit of the environmental control system 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...

 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
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 on a wing's leading edge can freeze. This build-up of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 adds weight and changes the shape of the wing, causing a degradation in performance, and possibly a critical loss of control or lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

. To prevent this, warm bleed air is pumped through the inside of the wing's leading edge. This heats up the metal, preventing 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.

Recent developments in civil aircraft

Bleed air systems have been in use for several decades in passenger jets. Boeing
Boeing Commercial Airplanes
Boeing Commercial Airplanes designs, assembles, markets and sells large commercial jet aircraft and provides product-related maintenance and training to customers worldwide...

 announced that its new aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

, the 787
Boeing 787
The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use...

 would operate without use of bleed air (and the two engines proposed for the aircraft, the General Electric GEnx
General Electric GEnx
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...

 and the Rolls-Royce
Rolls-Royce plc
Rolls-Royce Group plc is a global power systems company headquartered in the City of Westminster, London, United Kingdom. It is the world’s second-largest maker of aircraft engines , and also has major businesses in the marine propulsion and energy sectors. Through its defence-related activities...

 Trent 1000
Rolls-Royce Trent
Rolls-Royce Trent is the name given to a family of high bypass turbofan aircraft engines manufactured by Rolls-Royce plc. All are developments of the RB211 with thrust ratings of . Versions of the Trent are in service on the Airbus A330, A340, A380, Boeing 777, and 787, and variants are in...

, are designed with this in mind). This represents a departure from traditional aircraft design, and proponents state that 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.

In a bleed air system, air is compressed to several atmospheres by the engines, only to then be cooled and expanded (energy "lost") to a narrow pressure and temperature margin (requires complex systems to regulate temperature and pressure) to be piped around the plane, only then to be cooled or expanded again (energy "lost") to roughly one atmosphere of pressure. Additionally, eliminating bleed air reduces the aircraft's mass by removing a whole series of ducts, valves, heat exchangers and other heavy, maintenance intensive equipment.

The APU (auxiliary power unit) no longer needs 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. To pressurize the cabin, electric air compressors are used. 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
Air cycle machine
An air cycle machine is the refrigeration unit of the environmental control system 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...

 (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, and where the outside air is typically very cold, the ACM packs can be bypassed entirely, saving even more energy. Since no bleed air is taken from the engines for the cabin, 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.

Because the aircraft has no bleed air powered engine starters, and no requirement for bleed air for cabin pressurization during flight, there is no need to have the APU drive a load compressor to supply bleed air to the aircraft during a period of engine loss, or on the ground at the start of each flight. This simplifies aircraft design, eliminates another maintenance requirement and saves weight.

In contrast, eliminating bleed air creates a requirement for another source of energy for cabin pressurization, anti-ice/de-ice systems, and other functions previously covered by bleed air. The other source is electricity from large generators fitted to the main engines and APU. Therefore, from a systems point of view, this approach may potentially be less efficient. Rather than high pressure air from the engine (pneumatic energy) being used directly to pressurize the cabin, the pneumatic energy is converted to mechanical energy by the engine itself, and this shaft horsepower (mechanical energy) is taken from the engine to drive a generator (electrical energy) that is then used to drive a motor (mechanical energy) that is used to drive a compressor (pneumatic energy). Energy is lost at each conversion step, but despite all of these conversions, Boeing still expects a net energy savings. In the 787, the compressor motor is driven by the same power converters that are used to drive the electric engine starters for major weight savings.

While the 787 is considered a "bleedless" aircraft, a minor amount of bleed air is still used for engine pod intake de-ice. The amount of bleed air requirement for engine pod de-ice is so small, and since engine pod de-ice is not used for the entire flight, the engines are designed as if there were basically no typical bleed air requirements.

Airbus
Airbus
Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....

 does not currently (as of November 2007) have any plans to eliminate bleed air from its 787 competitor, the A350
Airbus A350
The Airbus A350 is a family of long-range, wide-body jet airliners under development by European aircraft manufacturer Airbus.A consortium originally comprising European aerospace companies from the UK, France, Spain and West Germany, Airbus is now fully owned by EADS and since 2001 has been known...

, and is improving its technology by improving the quality of cabin air and reducing the amount of needed bleed air and therefore increasing efficiency. The use of improved bleed air technology improves air quality in an aircraft pressure cabin at the same time. Airbus has received many patents in the last two decades for improving the efficiency of bleed air and improving the quality and security of cabin air.

Controversy

Certain neurological and respiratory ill health effects have been linked anecdotally to exposure to contaminated bleed air on commercial and military aircraft—the alleged associated long-term illness is known as aerotoxic syndrome
Aerotoxic syndrome
Aerotoxic syndrome is a term describing the alleged short-term and long-term ill-health effects that are attributed to exposure to cabin air that has been contaminated with atomized engine oils and other chemicals. The documentary film Welcome Aboard Toxic Airlines suggests the term was first...

. One potential contaminant is tricresyl phosphate
Tricresyl phosphate
Tricresyl phosphate, abbreviated TCP, is an organophosphate compound that is used as a plasticizer and diverse other applications. It is a colourless, viscous liquid, although commercial samples are typically yellow...

. Academic research is ongoing, such as that by University of New South Wales
University of New South Wales
The University of New South Wales , is a research-focused university based in Kensington, a suburb in Sydney, New South Wales, Australia...

, Cranfield University
Cranfield University
Cranfield University is a British postgraduate university based on two campuses, with a research-oriented focus. The main campus is at Cranfield, Bedfordshire and the second is the Defence Academy of the United Kingdom based at Shrivenham, Oxfordshire. The main campus is unique in the United...

, University of British Columbia
University of British Columbia
The University of British Columbia is a public research university. UBC’s two main campuses are situated in Vancouver and in Kelowna in the Okanagan Valley...

, University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

, University College London
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...

 (Sarah Mackenzie Ross), but no formal causal relationship
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 has been established yet by researchers.

A number of lobbying groups have been set up to raise public awareness, petition on behalf of passengers and crew, and to try to influence regulators. These groups include the Aviation Organophosphate Information Site (AOPIS) (2001), the Global Cabin Air Quality Executive (2006) and the UK-based Aerotoxic Association
Aerotoxic Association
The Aerotoxic Association was set up on 18 June 2007 at the British Houses of Parliament by Captain John Hoyte. The association is run by a group of airline pilots who believe they were made ill by the cabin air on jet aircraft, with the assumption that the bleed air was contaminated with engine oil...

 (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.

See also

  • Aerotoxic syndrome
    Aerotoxic syndrome
    Aerotoxic syndrome is a term describing the alleged short-term and long-term ill-health effects that are attributed to exposure to cabin air that has been contaminated with atomized engine oils and other chemicals. The documentary film Welcome Aboard Toxic Airlines suggests the term was first...

  • GCAQE
  • Welcome Aboard Toxic Airlines
    Welcome Aboard Toxic Airlines
    Welcome Aboard Toxic Airlines is a 2007 British investigative documentary film directed and produced by former airline captain Tristan Loraine on the subject of so-called aerotoxic syndrome.-Plot:...

  • British Air Line Pilots Association
    British Air Line Pilots Association
    The British Air Line Pilots' Association represents well over 75% of all the fixed wing pilots and helicopter aircrew based in the UK - as well as many working overseas. The Association has a membership of over 10,000 professional flight crew, working in companies large and small...

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