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. Additionally, bleed air is used to keep critical parts of the aircraft (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, 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 (APU) or an external huffer 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 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.
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 cooled by cold outside air. It is then fed to an air cycle machine 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 on a wing's leading edge can freeze. This build-up of ice 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, 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.
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