Bleed Air
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Bleed air is
compressed air Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes, and is used for power tools such as air hammers, drills, wrenches, and o ...
taken from the compressor stage of a
gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
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 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 ...
,
pneumatic actuator A pneumatic control valve actuator converts energy (typically in the form of compressed air) into mechanical motion. The motion can be rotary or linear, depending on the type of actuator. Principle of operation A Pneumatic actuator mainly co ...
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". Bleed air is valuable in an aircraft for two properties: high
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
and high
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
(typical values are 200–250 °C and 275 kPa (40 PSI), for regulated bleed air exiting the engine pylon for use throughout the aircraft).


Uses

In civil aircraft, bleed air's primary use is to provide pressure for the aircraft cabin by supplying air to the
environmental control system In aeronautics, an environmental control system (ECS) of an aircraft is an essential component which provides air supply, Air conditioning, thermal control and cabin pressurization for the Aircrew, crew and passengers. Additional functions incl ...
. Additionally, bleed air is used to keep critical parts of the plane (such as the wing
leading edge The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
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, bleed air from an airplane engine is used to start the remaining engines.
Lavatory Lavatory, Lav, or Lavvy may refer to: *Toilet, the plumbing fixture *Toilet (room), containing a toilet *Public toilet *Aircraft lavatory, the public toilet on an aircraft *Latrine, a rudimentary toilet *A lavatorium, the washing facility in a mon ...
water storage tanks are pressurized by bleed air that is fed through a
pressure regulator A pressure regulator is a valve that controls the pressure of a fluid or gas to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setti ...
. When used for
cabin pressurization 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 ...
, the bleed air from the engine must first be cooled (as it exits the compressor stage at temperatures as high as 250 °C) by passing it through an air-to-air
heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
cooled by cold outside air. It is then fed to an
air cycle machine 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 ...
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
intake An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...
s. 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 Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...
. 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 A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft ...
in the Harrier family of military aircraft.


Contamination

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 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. Potential sources of contamination Modern jetliners have envi ...
, but it is not a medically recognized syndrome. One potential contaminant is
tricresyl phosphate Tricresyl phosphate (TCP), is a mixture of three isomeric organophosphate compounds most notably used as a flame retardant and as a plasticizer in manufacturing for lacquers and varnishes and vinyl plastics. Pure tricresyl phosphate is a colourles ...
. 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 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 ...
(2007). Cabin Environment Research is one of many functions of the ACER Group, but their researchers have not yet established any
causal relationship Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the ca ...
. 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." 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 Toxic encephalopathy is a neurologic disorder caused by exposure to neurotoxic organic solvents such as toluene, following exposure to heavy metals such as manganese, as a side effect of melarsoprol treatment for African trypanosomiasis, adverse ef ...
(brain damage) from a fume event condemned the airline industry's obstructionism around fume events. In July 2015, pilots on a
Spirit Airlines Spirit Airlines Inc. (stylized as spirit) is a major ultra-low-cost U.S. carrier headquartered in Miramar, Florida, in the Miami metropolitan area. Spirit operates scheduled flights throughout the United States, the Caribbean and Latin Ameri ...
flight were partially incapacitated by fumes in bleed air.


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 The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes. After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
, 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.”


Benefits

A bleedless aircraft achieves
fuel efficiency Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, wh ...
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 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 ...
(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.


See also

*
Aerotoxic syndrome 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. Potential sources of contamination Modern jetliners have envi ...
*
Cabin pressurization 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 ...
*
Environmental control system (aircraft) In aeronautics, an environmental control system (ECS) of an aircraft is an essential component which provides air supply, Air conditioning, thermal control and cabin pressurization for the Aircrew, crew and passengers. Additional functions incl ...
*
Ice protection system 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 ...


References

{{DEFAULTSORT:Bleed Air Aerospace engineering Jet engines Occupational safety and health
Bleed air 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. Lo ...