Environmental control system (aircraft)

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This article is about systems on aircraft. For systems on spacecraft, see Life support system.
Control panel for a Boeing 737-800 ECS




Air supply[edit]

Main article: Bleed air

On most jetliners, air is supplied to the ECS by being "bled" from a compressor stage of each gas turbine engine, upstream of the combustor. The temperature and pressure of this "bleed air" varies widely depending upon which compressor stage and the RPM of the engine.

A "manifold pressure regulating shut-off valve" (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems. This flow restriction results in efficiency losses. To reduce the amount of restriction required, and thereby increase efficiency, air is commonly drawn from two bleed ports (three on the Boeing 777).

When the engine is at low thrust, the air is drawn from the "high pressure bleed port." As thrust is increased, the pressure from this port rises until "crossover," where the "high pressure shut-off valve" (HPSOV) closes and air is thereafter drawn from the "low pressure bleed port."

To achieve the desired temperature, the bleed-air is passed through a heat exchanger called a "pre-cooler." Air from the jet engine fan is blown across the pre-cooler, which is located in the engine strut. A "fan air modulating valve" (FAMV) varies the cooling airflow, and thereby controls the final air temperature of the bleed air.

Cold air unit[edit]

The cold air unit (CAU), or "air conditioning pack" is usually an air cycle machine (ACM) cooling device. Some aircraft, including early 707 jetliners, used vapor-compression refrigeration like that used in home air conditioners.


On most jetliners, the A/C packs are located in the "wing to body fairing" between the two wings beneath the fuselage. On some jetliners (Douglas Aircraft DC-9 Series) the A/C packs are located in the tail. The A/C packs on the McDonnell Douglas DC-10/MD-11 and Lockheed L-1011 are located in the front of the aircraft beneath the flight deck. Nearly all jetliners have two packs, although larger aircraft such as the Boeing 747, Lockheed L-1011, and McDonnell-Douglas DC-10/MD-11 have three.






Ram air system[edit]



Air distribution[edit]



Air from the "mix manifold" is directed to overhead distribution nozzles [1] in the various "zones" of the aircraft. Temperature in each zone may be adjusted by adding small amounts of "trim air", which is low-pressure, high-temperature air tapped off the A/C pack upstream of the TCV. Air is also supplied to individual gaspers: small, circular vents above each passenger seat that can be adjusted by passengers for their personal comfort. A revolving control on the vent can be turned to adjust ventilation between no air output at all and a fairly substantial breeze.

Gasper over passenger seats of a Boeing 737-800

Gaspers usually receive their air from the air conditioning packs aboard the aircraft, which in turn receive compressed, clean air from the compressor stages of the aircraft's jet engines or when on the ground from the auxiliary power unit (APU) or a ground source. A master control for gaspers is located in the cockpit, and gaspers may be temporarily turned off during certain phases of flight, when the load on the engines from bleed-air demands must be minimized (e.g. take-off and climb).


Main article: Cabin pressurization
Outflow and pressure relief valve on a Boeing 737-800






A cabin humidity control system may be added to the ECS of some aircraft to keep relative humidity from extremely low levels, consistent with the need to prevent condensation.[4] Furthermore the Boeing 787 and Airbus 350, by using more corrosion-resistant composites in their construction, can operate with a cabin relative humidity of 16% on long flights.

Health concerns[edit]

Main article: Aerotoxic Syndrome

The bleed air comes from the engines but is "bled" from the engine upstream of the combustor. Air cannot flow backwards through the engine except during a compressor stall (essentially a jet engine backfire), thus the bleed air should be free of combustion contaminants from the normal running of the aircraft's own engines.

However, on occasions carbon seals can leak oil (containing potentially hazardous chemicals) into the bleed air, in what is known in the industry as a "fume event".[5] This is generally dealt with quickly since failed oil seals will reduce the engine life.



  1. ^ Eitel, Elisabeth. CFD Software Models How Moving Parts Affect Aircraft-Cabin Airflow | Machine Design Magazine, 6 May 2014.
  2. ^ Niren Laxmichand Nagda (Ed): Air Quality and Comfort in Airliner Cabins. ASTM International (2000) ISBN 978-0-8031-2866-8.
  3. ^ "Cabin Air Quality." Statement of William W. Nazaroff, Ph.D. Professor of Environmental Engineering, University of California, Berkeley and Member, Committee on Air Quality in Passenger Cabins of Commercial Aircraft. (June 5, 2003)
  4. ^ "CTT Systems AB receives cabin humidity control system order from Jet Aviation AG". Airline Industry Information, (March 5, 2007)
  6. ^ Bagshaw, Michael (September 2008). "The Aerotoxic Syndrome". European Society of Aerospace Medicine. Retrieved December 31, 2012. 
  7. ^ Select Committee on Science and Technology (2000). "Chapter 4: Elements Of Healthy Cabin Air". ﻿Science and Technology - Fifth Report﻿ (Report). House of Lords. http://www.publications.parliament.uk/pa/ld199900/ldselect/ldsctech/121/12107.htm. Retrieved 2010-07-05.

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