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Bleed Air Systems
The design of most turbojet and turboprop powered aircraft incorporates a bleed air system. A bleed air system uses a network of ducts, valves and regulators to conduct medium to high pressure air, "bled" from the compressor section of the engine(s) and APU, to various locations within the aircraft. There it is utilized for a number of functions inclusive of:
- air conditioning
- engine start
- wing and engine anti-ice systems
- water system pressurisation
- hydraulic system reservoir pressurisation
- boundary layer separation enhancement
Bleed Air Extraction
Bleed air is extracted from the compressor of the engine or APU. The specific stage of the compressor from which the air is bled varies by engine type. In some engines, air may be taken from more than one location for different uses as the temperature and pressure of the air is variable dependant upon the compressor stage at which it is extracted. Bleed air typically has a temperature of 200 – 250 degrees C. and a pressure of approximately 40 PSI exiting the engine pylon.
Bleed air is routed to the air conditioning packs where it is filtered and then cooled using an expansion process. The temperature of the air is regulated using uncooled bleed air and the humidity of the mixture is adjusted prior to introducing the air into the aircraft cabin. Temperature controllers in the flight deck and cabin allow adjustment of the target temperature and thermostats provide feedback to the packs to demand an increase or decrease in the output temperature.
Bleed air, extracted from either the Auxiliary Power Unit or another operating engine is used to power an air turbine starter motor to start the engine. The primary advantage of an air turbine starter is that a given amount of torque can be produced by a smaller and lighter unit than would be the case if it was electrically or hydraulically powered.
Water System / Hydraulic Reservoir Pressurisation
Bleed air is often utilized to pressurise the potable water holding tank eliminating the requirement for a pump to feed the water to the galleys and lavatories. Similarly, bleed air is used to pressurise the hydraulic system reservoirs of many aircraft reducing the likelihood of pump cavitation and the resulting loss of system pressure.
Boundary Layer Enhancement (Blown Flaps)
Although its current use is very limited, bleed air has been used in the past, mainly in military applications, to enhance boundary layer energy. In a conventional blown flap, a small amount of bleed air is piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and the maximum lift coefficient by delaying boundary layer separation from the airfoil.
The major threat associated with a bleed air system is the potential risk of a leak resulting from loss of system integrity. A bleed air leak can lead to loss of system function, overheat or even fire. This topic is covered in detail in the article entitled Bleed Air Leaks.
Aircraft design has featured bleed air systems for a number of decades. However, with the introduction of the B787, Boeing has incorporated a new no-bleed systems architecture that eliminates the traditional pneumatic system and bleed manifold. Most functions formerly powered by bleed air such as the air-conditioning packs and wing anti-ice systems are now electrically powered. According to Boeing, the no-bleed systems architecture offers operators a number of benefits, including:
- Improved fuel consumption due to a more efficient secondary power extraction, transfer, and usage.
- Reduced maintenance costs due to elimination of the maintenance-intensive bleed system.
- Improved reliability due to the use of modern power electronics and fewer components in the engine installation.
- Expanded range and reduced fuel consumption due to lower overall weight.
Accident & Incidents
Events held on the SKYbrary A&I database which include reference to the bleed air system include:
- B737 en-route, Glen Innes NSW Australia, 2007 (On 17 November 2007 a Boeing 737-700 made an emergency descent after the air conditioning and pressurisation system failed in the climb out of Coolangatta at FL318 due to loss of all bleed air. A diversion to Brisbane followed. The Investigation found that the first bleed supply had failed at low speed on take off but that continued take off had been continued contrary to SOP. It was also found that the actions taken by the crew in response to the fault after completing the take off had also been also contrary to those prescribed.)
- B752, en-route, North Sea, 2006 (On 22 October 2006 a blue haze was observed in the passenger cabin of a Boeing 757-200, operated by Thomsonfly, shortly after reaching cruise altitude on a scheduled passenger flight from Newcastle to Larnaca. A precautionary diversion was made to London Stansted, where an emergency evacuation was carried out successfully.)
- A320, en-route, north of Öland Sweden, 2011 (On 5 March 2011, a Finnair Airbus A320 was westbound in the cruise in southern Swedish airspace after despatch with Engine 1 bleed air system inoperative when the Engine 2 bleed air system failed and an emergency descent was necessary. The Investigation found that the Engine 2 system had shut down due to overheating and that access to proactive and reactive procedures related to operations with only a single bleed air system available were deficient. The crew failure to make use of APU air to help sustain cabin pressurisation during flight completion was noted.)
- B738, Glasgow UK, 2012 (On 19 October 2012, a Jet2-operated Boeing 737-800 departing Glasgow made a high speed rejected take off when a strange smell became apparent in the flight deck and the senior cabin crew reported what appeared to be smoke in the cabin. The subsequent emergency evacuation resulted in one serious passenger injury. The Investigation was unable to conclusively identify a cause of the smoke and the also- detected burning smells but excess moisture in the air conditioning system was considered likely to have been a factor and the Operator subsequently made changes to its maintenance procedures.)
- A333, en-route, south of Moscow Russia, 2010 (On 22 December 2010, a Finnair Airbus A330-300 inbound to Helsinki and cruising in very cold air at an altitude of 11,600 metres lost cabin pressurisation in cruise flight and completed an emergency descent before continuing the originally intended flight at a lower level. The subsequent Investigation was carried out together with that into a similar occurrence to another Finnair A330 which had occurred 11 days earlier. It was found that in both incidents, both engine bleed air systems had failed to function normally because of a design fault which had allowed water within their pressure transducers to freeze.)