Bleed Air Leaks

Definition

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 a turbine engine, 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 from the compressor, at an intermediate stage or just after the last stage, but before the combustors. The use of bleed air is common in jet engine powered aircraft inclusive of turbojetturbofan and turboprop installations. Bleed air is useful 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 and, in some cases, it is used as the motive power for pneumatically driven hydraulic pumps.

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
  • components to overheat
  • damage to aircraft structures
  • inflight fire.

Even after the the bleed air leak has been secured using the appropriate ECAMQRH or AFM procedure, secondary effects of the original fault may occur. Isolating part of the bleed air system will inevitably lead to some degradation in the operation of other aircraft systems such as:

  • pneumatically operated control surfaces
  • air driven hydraulic pumps
  • air conditioning/presurisation systems
  • anti-icing systems.

Managing the remainder of the flight with the loss of some or all of these systems will require careful thought and planning. Having a comprehensive knowledge of the pneumatic systems in the aircraft will help in the decision making process. It is essential that the pilots understand what is working and what is not as well as the consequent limitations to the operation.

Defences

Some combination of gauges and warning systems is incorporated into the bleed air system to allow the pilots to monitor the normal function of the system and to provide audio and/or visual warning in the event of an overhear or failure. Valves are incorporated into the system to provide the means to automatically or manually isolate parts of the bleed air manifold or individual components in the event of a failure. Some of these defenses include:

  • Cockpit Gauges - allow the pilots to monitor bleed air manifold temperature and pressure.
  • Overheat Detectors - located in close proximity to bleed air ducts. In the event of a bleed air leak from a ruptured duct, the overheat detector will cause a warning to be generated on the flight deck.
  • Bleed Air Shut-off Valves - 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 Systems - detect the loss of pressure caused by a duct failure and generate a warning on the flight deck.
  • Fire Wall Bleed Air Shut-off Valves - allow the bleed air from an engine to be isolated from the rest of the aircraft. It is typically closed when the Engine Fire checklist is actioned. Closing the fire wall bleed air valve prevents contamination of the bleed air system by the failed engine and, if the fire warning was caused by a ruptured bleed air duct within the engine, prevents the bleed air system from perpetuating the warning.

The electronic and mechanical defenses, as listed above, are essential to the timely detection and successful containment of a bleed air leak. However, many emergency or abnormal checklists for bleed air faults require some post action analysis to assess whether the action taken has been successful. A critical part of that analysis is a sound understanding, by the pilots, of the pneumatic system and all of its associated functions and components. If the isolation has not been successful, diversion should be initiated and an appropriate balance struck between the time spent on analysis and the need to get the aircraft on the ground as quickly as possible. Even when the isolation is successful, the pilots need to consider how the failure will affect the remainder of the flight. The impact of the loss of all or part of the bleed air system as it affects their particular aircraft type must be examined. Depending upon aircraft type, the analysis might consider items such as:

  • Icing - are anti-icing systems affected by the failure? Are there specific AFM limitation to be considered?
  • Pressurisation - can the planned altitude be maintained?
  • Approach, Landing, Go-Around - does the failure in any way impact upon extension or retraction of landing gear, high lift or deceleration devices?

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. The notes and cautions associated with the checklist procedure advise that "icing conditions must be avoided". Moderate mixed icing has been reported by aircraft in descent at the planned destination. The crew elects to divert to their alternate where there is some cloud cover but no icing is forecast or has been reported.

Contributing Factors

Aircraft wiring is often routed in proximity to pneumatic ducts. A bleed air leak from a compromised duct can melt the insulation of these wires causing short circuits and potentially result in the generation of a number of false warnings. These multiple warnings may 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:

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