The Importance of Bleed Air Systems
Bleed air is the air that is released from the compressor stage of an engine. When released, the bleed air is approximately 200-250°C and at a medium-high pressure of approximately 40 psi. This heat and pressure means bleed air can be used to power many different aircraft systems. As bleed air is a readily available energy source (as long as the engines are working, bleed air will be available), it is constantly used throughout modern aircraft. Bleed air’s two main benefits are its heat, which is used for things like anti-ice and de-icing systems, and pressure, which is used for things like cabin pressurization and air conditioning.
The first system where bleed air is used is anti- or de-icing systems. Icing conditions can exist both on the ground and while airborne, but the use of bleed air system can mitigate the risk in both situations. Icing can occur at temperatures between 10°C and -50°C if moisture such as rain or droplets in a cloud is present. There are two methods by which bleed air is used in anti- and de-icing systems: air tubes and de-icing boots. By using air tubes in the wings, tail surfaces, and engine inlets to re-route bleed air, aircraft surfaces can be heated to temperatures above freezing, melting away any ice and preventing the formation of more. This is the most common method of ice protection for big jets. Bleed air is also used in de-icing boots, which are layers of rubber on the leading edge of the wing that inflate with bleed air to alter the shape of the wing and break off unwanted ice.
Perhaps surprisingly, bleed air is also used to help the aircraft’s engines start. Normal turbofan engines are started prior to taxiing and can take two to three minutes to come up to speed. In order to carry this out, the engine must have an operational auxiliary power unit supplying both electricity and bleed air. When startup begins, bleed air is funneled from the APU to the accessory gearbox to help rotate the various engine shafts. Once the shafts reach a speed where they no longer need bleed air to rotate, engine start is complete.
Finally, the most important use of bleed air is in cabin pressurization. Pressurized air within the cabin allows those onboard to breathe without masks, so the already-pressurized bleed air from the engine is a great source. However, as stated, bleed air can be up to 250°C, meaning it must be cooled before being directed into the cabin. This is done through the use of an air-to-air heat exchanger that uses cold air from outside the aircraft and passes it through the hot bleed air until an appropriate temperature is reached. Both engines are used to supply bleed air, though there are other sources available depending on what stage of flight the aircraft is in. For instance, while still on ground, many airports provide bleed air through pipes. Additionally, the auxiliary power unit can be used to provide bleed air from the rear of the aircraft. Failure of the bleed air system can lead to cabin depressurization and force an emergency descent, so its proper function is critical to flight.