Aircraft Pneumatic Bleed-Air Supply Systems
The pneumatic system supplies pressurized air for various aircraft functions. In turbine aircraft, this is primarily "bleed air" extracted from the engines.
Bleed Air System
Bleed air is air that is tapped from the compressor stage of a turbine engine or the APU.
- Source:
- It is always extracted from the compressor.
- The specific extraction point varies, but it is typically taken from the 5th stage (Low pressure stage) of the High Pressure Compressor.
- Pressure Measurement: Bleed-air pressure gauges allow pilots to monitor the system. This pressure is measured relative to atmospheric static pressure.
- Primary Uses:
- Pressurization: Supplying air to the cabin (controlled by outflow valves).
- Air Conditioning: Providing cooling (bleed air is hot and must be cooled).
- Ice Protection: Thermal anti-icing.
- Hydraulic Reservoir Pressurization: To provide positive fluid pressure at the pump inlet and prevent air bubbles (cavitation) at high altitudes.
Pneumatic Systems in Piston Aircraft
Piston aircraft also utilize pneumatic systems, though the source and applications differ. Common uses include:
- Cabin heating
- De-icing systems (inflatable boots)
- Gyroscopic instruments
- Note: Electrical power generation and hydraulic pumps typically do not use the pneumatic system. Pitot-static instruments rely on ram/static air, not system pneumatic pressure.
Electric Air Conditioning Systems
Some modern aircraft (e.g., Boeing 787) use electrically driven air conditioning instead of traditional bleed air systems.
- Advantages:
- Reduced Fuel Consumption: Due to more efficient secondary power extraction and transfer.
- Reduced Maintenance Costs: Eliminates the maintenance-intensive bleed air system components.
- Lower Weight: Contributes to expanded range and further fuel savings.
- Improved Reliability: Uses modern power electronics with fewer engine components.