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Aircraft Bleed Air Systems
From SKYbrary Wiki
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:
- B735, en-route, SE of Kushimoto Wakayama Japan, 2006 (On 5 July 2006, during daytime, a Boeing 737-500, operated by Air Nippon Co., Ltd. took off from Fukuoka Airport as All Nippon Airways scheduled flight 2142. At about 08:10, while flying at 37,000 ft approximately 60 nm southeast of Kushimoto VORTAC, a cabin depressurization warning was displayed and the oxygen masks in the cabin were automatically deployed. The aircraft made an emergency descent and, at 09:09, landed on Chubu International Airport.)
- A332, vicinity Perth Australia, 2014 (On 9 June 2014, a 'burning odour' of undetermined origin became evident in the rear galley of an Airbus A330 as soon as the aircraft powered up for take off. Initially, it was dismissed as not uncommon and likely to soon dissipate, but it continued and affected cabin crew were unable to continue their normal duties and received oxygen to assist recovery. En route diversion was considered but flight completion chosen. It was found that the rear pressure bulkhead insulation had not been correctly refitted following maintenance and had collapsed into and came into contact with APU bleed air duct.)
- 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.)
- A320, vicinity London Heathrow UK, 2019 (On 23 September 2019, the flight crew of an Airbus A320 on approach to London Heathrow detected strong acrid fumes on the flight deck and after donning oxygen masks completed the approach and landing, exited the runway and shut down on a taxiway. After removing their masks, one pilot became incapacitated and the other unwell and both were taken to hospital. The other occupants, all unaffected, were disembarked to buses. The very comprehensive investigation was unable to establish the origin of the fumes but did identify a number of circumstantial factors which corresponded to those identified in previous similar events.)
- A320, en-route, northeast of Granada Spain, 2017 (On 21 February 2017, an Airbus A320 despatched with the APU inoperative experienced successive failures of both air conditioning and pressurisation systems, the second of which occurred at FL300 and prompted the declaration of a MAYDAY and an emergency descent followed by an uneventful diversion to Alicante. The Investigation found that the cause of the dual failure was likely to have been the undetectable and undetected degradation of the aircraft bleed air regulation system and whilst noting a possibly contributory maintenance error recommended that a new scheduled maintenance task to check components in the aircraft type bleed system be established.)