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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 plane (such as the wing leading edges ) ice-free.
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.
By the time the cold outside air has reached the bleed air valves, it has been heated to around 200 °C (392 °F). The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight. Piston-engine aircraft require an additional compressor, see diagram ...
The control panel that controls cabin pressurisation and bleed air distribution on a Boeing 737-800. A fume event occurs when bleed air used for cabin pressurisation and air conditioning in a pressurised aircraft is contaminated by fluids such as engine oil, hydraulic fluid, anti-icing fluid, and other potentially hazardous chemicals.
The compressed air can be supplied from an on-board auxiliary power unit (APU), a portable gas generator used by ground crew or by cross feeding bleed air from a running engine in the case of multi-engined aircraft. [19] The Turbomeca Palouste gas generator was used to start the Spey engines of the Blackburn Buccaneer.
Cabin Pressure and Bleed Air Control Panels on a Boeing 737-800. Modern jetliners have environmental control systems (ECS) that manage the flow of cabin air. Outside air enters the engines and is compressed in the forward section of the engine, prior to the combustion section, ensuring no combustion products can enter the cabin.
Bleed air systems are used by most large aircraft with jet engines or turboprops. Hot air is "bled" off one or more engines' compressor sections into tubes routed through wings, tail surfaces, and engine inlets. Spent air is exhausted through holes in the wings' undersides.
Supplying bleed air to the aircraft decreases the efficiency of the engine because it has been compressed, but then does not contribute to producing thrust. Compressor types used in turbojets were typically axial or centrifugal. Early turbojet compressors had low pressure ratios up to about 5:1.