Bleed Air Leaks

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.

Effects

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.

Defences

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.

Typical Scenarios

• 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.

Contributing Factors

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.

Fonte: SkyBrary