Bleed air

Bleed air in jet turbines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. This compressed air can be used in many different ways, from de-icing to pressurising the cabin to pneumatic actuators. However, bleed air is quite hot and if being used in the cabin or other low temperature areas it must be cooled, even refrigerated. Bleed air is valuable in an aircraft for two properties: its high temperature and its high pressure.

Since modern jet turbines use multiple compressor stages, some newer engines for new aircraft designs have the bleed air inlet between compressor stages to reduce the temperature and reduction in the need for compressed air in more electric aircraft.

Merits of bleed air

In civil aircraft, its 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 aircraft (such as the wing leading edges) ice-free.

When used for cabin pressurization, the air from the engine must first be cooled (as it exits the engine at temperatures as high as 1000 °C) by passing the bleed air through an air-to-air heat exchanger cooled by cold outside air. It is then fed to an air conditioning unit which regulates the temperature and flow of air into the cabin, keeping the environment comfortable.

A similar system is used for wing de-icing. Under disadvantageous conditions, water droplets condensing on a wing's leading edge can freeze at the ambient temperatures experienced during flight. This build-up of ice will change the shape of the wing, causing a degradation in performance, and possibly a fatal loss of lift. 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.

Recent developments in civil airplanes

Bleed air systems have been in use for several decades in passenger jets. Recently, Boeing announced that its new airplane, the 787 would operate without use of bleed air (and the two engines proposed for the airplane, the General Electric GEnx and the Rolls-Royce Trent 1000, are designed with this in mind). This represents a departure from traditional airplane design, and proponents argue that eliminating bleed air improved engine efficiency, as there is no loss of mass flow of air (and therefore energy) from the engine, leading to lower fuel consumption. Additionally, proponents of "bleed-air-less designs" argue that eliminating bleed air reduces the aircraft's mass by eliminating a whole series of pumps, heat exchangers and other heavy equipment. Lastly, they argue that this design improves safety as superheated air (often at temperatures approaching 1200 °C) is confined to the engine core, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak of superheated air could damage surrounding systems.

However, opponents claim that eliminating bleed air requires finding alternative methods of providing cabin heating, de-icing and other functions previously covered by bleed air, which require additional systems which take up space, weight, and electrical energy. They therefore argue that this approach is less efficient from an overall point of view (taking the entire airplane into consideration and not just the engines), as it necessarily involves drawing in very cold ambient air and heating it, a very energy-intensive process.

Airbus does not currently (as of November 2004) have any plans to eliminate bleed air from its 787 competitor, the A350, while Boeing is actively pursuing this technology, touting it as one of the main advantages of its design.