Bleed air
Bleed air is out of the sheath current (secondary) and / or the core stream ( primarily ) an engine or auxiliary power unit air removed for regulation and control of the engine or entire aircraft systems. Candidate for the use of bleed air, especially the gas turbine are, for example a turbofan or an auxiliary engine. The use internationally in aviation term for this is Bleed Air
Bleed air for engine control and monitoring ( Engine Air System )
Cooling of jet engine parts
Air from the ducted fan (Fan Air) is used for cooling of components such as the engine and generator oil, the electronic control of the turbine housing or the igniter ( Igniter box). It also serves to cool the high pressure bleed air that has to be cooled by up to 750 ° C to about 180 ° C.
Compressor air from the higher pressure region ( approximately twelfth stage compressor ) is used to cool the rotor and stator blades of the high pressure turbine.
Control of jet engines
High-pressure bleed air is prior to the control variable stators and used in the compressor to avoid stalls ( eng. stall ), such as in the Pratt & Whitney PW4000 on the 5th to 7th stage compressor.
The theory behind this phenomenon is that of the stall to a moving air through the airfoil.
Feeding into aircraft systems (Air Supply Distribution System)
Bleed air comes in different systems of an aircraft for use. Here it is used among other things for thermal regulation and pressure supply of the airframe ( cabin pressure ). Also hydraulic and water tanks are kept under pressure by means of the bleed air, for example to prevent failure of the pump. Auxiliaries can bleed air means in the event of an overload of the primary system, such as the engine driven hydraulic pumps guarantee the operability of the system.
Pros and Cons
Bleed air is a simple and proven system that is composed of technically very simple to implement components.
Most serious drawback, however, is that increases in air bleed the fuel consumption and performance is reduced. At high starting power therefore the air bleed is switched off in order to have in the critical start-up phase the full engine power. The efficiency is poor, because the pressure and the temperature must be greatly reduced to prevent damage to the aircraft side bleed air system. The Boeing 787, therefore the engines are built without removal option for bleed air - one hopes that this will lower fuel consumption. The operation of the air conditioning and the auxiliary equipment works then by electricity, which is why the engines get much stronger to compensate generators.
In addition, there is criticism that poisoning can be caused by oil fumes when the air is fed unfiltered into the cabin due to a seal damage. This poisoning can lead to an aero toxic syndrome. In the past, there were multiple incidents with contaminated bleed air. 2006 of such incidents in the aviation authority were in England about 1050 and captured - unlike in Germany - also published. In February 2009 the Lufthansa admitted in an internal memo to its employees that it will come to an incident with oil vapors in a 2000 flights. German airlines are not obliged to inform their passengers about such events.
To avoid these risks, would bleed air -operated systems, such as the production of the cabin pressure as well as the de-icing work in other ways. In the use of electrical energy for these functions would be an electrically- driven compressor used for example for maintaining cabin pressure. In the conversion of mechanical energy into electrical energy and vice versa is not usable energy is lost, the compressed air drives the motor that drives the generator, the electrical energy, in turn, drives a compressor to compress air for the cabin.