Aircraft Bleed Air Systems
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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 (APU) 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:
- 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.)
- A332, Karachi Pakistan, 2014 (On 4 October 2014, the fracture of a hydraulic hose during an A330-200 pushback at night at Karachi was followed by dense fumes in the form of hydraulic fluid mist filling the aircraft cabin and flight deck. After some delay, during which a delay in isolating the APU air bleed exacerbated the ingress of fumes, the aircraft was towed back onto stand and an emergency evacuation completed. During the return to stand, a PBE unit malfunctioned and caught fire when one of the cabin crew attempted to use it which prevented use of the exit adjacent to it for evacuation.)
- 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.)
- B734, en-route, east northeast of Tanegashima Japan, 2015 (On 30 June 2015, both bleed air supplies on a Boeing 737-400 at FL370 failed in quick succession resulting in the loss of all pressurisation and, after making an emergency descent to 10,000 feet QNH, the flight was continued to the planned destination, Kansai. The Investigation found that both systems failed due to malfunctioning pre-cooler control valves and that these malfunctions were due to a previously identified risk of premature deterioration in service which had been addressed by an optional but “recommended” Service Bulletin which had not been taken up by the operator of the aircraft involved.)
- 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.)