Bleed air

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Bleed air is compressed air that is taken from the combustion air stream compressed in the turbo compressor of gas turbines for various purposes. The bleed air is branched off before the air flow enters the combustion chambers . For example, jet engines , propeller turbines , on-board auxiliary gas turbines (APUs) or even ground starters can be considered as suppliers of bleed air . The term used internationally in aviation for this is bleed air .

Feeding into aircraft systems (Air Supply Distribution System)

Bleed air is used in a wide variety of pneumatic systems in an aircraft. Among other things, it is used for air conditioning and pressure supply of the pressure cabin in the air conditioning packs . Bleed air from the APU or a ground starter is used to start the engines. The bleed air, heated by the heat of compression, is used for targeted heating (thermal anti-icing) of surfaces sensitive to icing such as engine inlets, nose edges of wings or tail units or slat flaps.

The Lockheed F-104 Starfighter had a boundary layer blowout by means of engine bleed air at the gaps in the landing flaps, the functionality of which was essential for the aircraft's ability to fly on approach .

Hydraulic and water tanks are also kept under pressure by means of bleed air, for example to prevent pumps from failing . Auxiliary units can use bleed air to guarantee the functionality of a system in the event of an overload of the primary systems such as the engine-driven hydraulic pumps .

Bleed air for engine regulation and control (Engine Air System)

Cooling of jet engine parts

Air from the bypass flow ( fan air ) is used to cool components such as the turbine and generator oil , the electronic engine control , the turbine housing or the igniter box . It is also used to cool the high-pressure bleed air, which has to be cooled from up to 750 ° C to around 180 ° C.

Compressor air from the higher pressure range (approximately twelfth compressor stage) is used to cool the rotor and stator blades of the high-pressure turbine, which is passed through cooling channels in the blades radially from the hub to the blade tip of each blade.

Control of jet engines

High-pressure bleed air, for example, for the pneumatic control of variable guide vanes in front and in the compressor for adaptation to the respective operating state and to prevent compressor surge and stalls (engl. Compressor stall ) is used, as in the Pratt & Whitney PW4000 on the 5th to 7th compressor stage .

Advantages and disadvantages

The use of bleed air in aircraft is a simple and proven system that is made up of components that are easy to implement from a technical perspective.

The extraction of a lot of bleed air from the engines increases their fuel consumption and lowers their performance. When starting , the bleed air extraction is switched off in order to have the full engine power available. The efficiency is poor, since the pressure and the temperature have to be greatly reduced in order to prevent damage to the bleed air system on the aircraft.

It is also criticized that the use of bleed air to pressurize pressurized cabins can lead to poisoning by oil vapors if the extracted air is contaminated with lubricating oil from the oil circuit due to a seal damage in the engine compressor . This poisoning can lead to an aerotoxic syndrome . In the past, there have been several incidents with contaminated bleed air. In 2006 more than 1050 such incidents were recorded by the aviation authority in Great Britain and - unlike in Germany - also published. In February 2009, Lufthansa admitted in an internal message to its employees that there was an oil vapor incident on one of 2000 flights. German airlines are (as of 2011) not obliged to inform their passengers about such events.

On May 7, 2014, the Federal Bureau of Aircraft Accident Investigation (BFU) published a study on the impact of so-called fume events (events of any kind with regard to odors, smoke or fog in the aircraft interior). This confirms that there have been health problems. In some cases, the formal prerequisite for a serious incident was met by putting on the oxygen masks in the cockpit or by a partial failure of a pilot . As a result, the BFU issued four safety recommendations in order to better scientifically investigate fume events , to report the events to the BFU, to standardize the approval criteria and to establish verification procedures for clean cabin air and to carry out studies on long-term illnesses.

Alternatives to the use of bleed air

In order to avoid the risks and disadvantages, the systems usually operated with bleed air, such as maintaining the cabin pressure and defrosting, have to function in a different way. When using electrical energy for these tasks, an electrically operated compressor would be used to maintain the cabin pressure.

In the Boeing 787 , for the first time, engines without an external bleed air option ( Rolls-Royce Trent 1000 or GEnx-1B ) are used and the usual pneumatic on-board systems are dispensed with - this is expected to reduce fuel consumption. The operation of the air conditioning, the auxiliary units, the de-icing and also the engine starter works completely electrically, which is why the engines have received significantly more powerful generators to compensate for this, and powerful on-board electronics are installed.


  • Approach for a new alternative to bleed air. In: Flug Revue No. 3/2018, pp. 68–71

Individual evidence

  1. Tim van Beveren: When neurotoxins get into the plane. In: The world . April 5, 2012, accessed September 25, 2014 .
  2. Study of reported events in connection with the quality of cabin air in commercial aircraft. (pdf) Federal Bureau of Aircraft Accident Investigation , May 7, 2014, accessed on May 26, 2014 .
  3. 787 No-Bleed Systems: Saving Fuel and enhancing operational efficiencies , description at , English, accessed on September 16, 2016