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Bleed Air Leaks
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|Category:||Fire Smoke and Fumes|
An aircraft Bleed Air Leak refers to the uncontrolled loss of bleed air from any part of the aircraft pneumatic system or from the services which utilize bleed air.
Bleed Air Systems
Bleed air in the context of turbine engines refers to compressed air which is taken from within the engine. The point at which the air is bled from the engine varies by engine type but is always tapped at the level of the compressor, at an intermediate stage or after the last stage, but before the combustors. Bleed air usage is common in virtually all jet turbine powered aircraft as well as in many turbo-prop powered aircraft. Bleed air is a useful commodity in an aircraft because of two properties: high temperature (typically 200 – 250 degrees C.) and moderate pressure (regulated to approximately 40 PSI exiting the engine pylon). This hot, compressed air can be used in many different ways. Typical uses include engine start, air conditioning and pressurization, engine and airframe de/anti-icing, pressurization of water reservoirs, hydraulic reservoirs and pneumatically powered actuators.
The uncontrolled loss of bleed air from the pneumatic system or from any of the pneumatically powered services has the potential to cause damage to aircraft wiring, overheat components, or even fire.
Overheat Detectors are located in close proximity to bleed air ducts. In the event of a bleed air leak caused from a ruptured duct, the overheat detector will cause a warning to be generated on the flight deck.
Bleed Air Shut-off Valves are located at various points in the pneumatic system. In the event of a failure, the shut-off valves can be used to isolate the failed portion of the system.
Bleed Air Monitoring Computers installed on some aircraft will detect the loss of system pressure caused by a duct failure and generate a warning on the flight deck.
The Fire Wall Bleed Air Shut-off Valve allows the bleed air from an engine to be isolated from the rest of the aircraft. It is typically closed when the pilots action the Engine Fire checklist. This prevents contamination of the bleed air system by the failed engine and, in the situation where the fire warning was caused by a ruptured bleed air duct within the engine, prevents the bleed air system from perpetuating the failure.
- A bleed air modulating valve in the right wing anti-ice system fails in the fully open position causing the anti-ice system to overheat. A flight deck warning is generated and the wing anti-ice system is turned off. The aircraft is descended to warmer air where icing is no longer a factor.
- The main pneumatic duct in the left wing suffers a catastrophic failure. The overheat detectors in proximity to the duct generate a warning on the flight deck. Checklist action is followed to close the fire wall bleed air shut-off valve on the left engine and the bleed air shut-off valve for the left wing isolating the leak.
Aircraft wiring is often routed in close proximity to pneumatic ducts. A bleed air leak from a compromised duct can quickly melt the insulation of these wires causing short circuits and potentially resulting in any number of false warnings being generated. These warnings have the potential to mask the actual failure. If the bleed air leak is allowed to persist, heat damage to the airframe structure or a fire is possible.
Accidents and Incidents
The following events involved problems with the bleed air system and/or bleed air leaks:
- B737 en-route, Glen Innes Australia, 2007 (AW HF) (On 17 November 2007 a Boeing 737-700 being operated by Virgin Blue on a scheduled passenger service from Coolangatta to Melbourne at night experienced a right hand engine bleed trip off during the take off, which was continued. A subsequent attempt at reset was not successful and once above FL170, above which APU air use was not permitted, only the left engine bleed air would be available for air conditioning and cabin pressurisation. The flight crew initially decided to cruise at a lower level than planned, FL250, but once there, icing conditions were encountered and the crew decided to continue the climb to FL350 to cruise clear of cloud.)
- B735, en-route, SE of Kushimoto Wakayama Japan, 2006 (AW) (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.)
- B738, Glasgow UK, 2012 (FIRE AW) (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.)
- A320, en route, north of Öland Sweden, 2011 (AW LOC HF) (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.)
- A333, En route, south of Moscow Russia, 2010 (LOC AW HF) (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.)
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