The A320 pneumatic system distributes high-pressure bleed air from engines, APU, or external ground supply to various aircraft systems. Understanding IP/HP bleed logic, pressure regulation at 45±5 PSI, precooler operation, crossbleed logic, and leak detection is essential for type rating.
Bleed Air Sources
- Engine Bleed Systems: IP stage (normal) or HP stage (backup)
- APU Bleed: Available on ground and in flight when APU >95%
- External HP Ground Supply: High-pressure ground cart connection
System Supervision
Bleed Monitoring Computer (BMC): One BMC supervises each engine bleed system. If one BMC fails, the opposite BMC takes over but automatic closure and leak detection cautions are lost for the failed side.
Engine Bleed - IP/HP Logic
The normal bleed source during stable engine operation is the intermediate pressure (IP) stage of the compressor.
HP Valve Operation
- HP Valve Closes When: Upstream pressure is low, excessive, or bleed valve commanded closed
- IP Check Valve: Prevents HP air from flowing back toward the IP stage
🔧 Pressure Regulation
Normal Delivery Pressure: Approximately 45 ± 5 PSI
Overpressure Valve Closes: If bleed pressure exceeds ~75 PSI
Bleed Valve Closure Logic
Pneumatic Closure
Bleed valve closes pneumatically if:
- Upstream pressure drops below 8 PSI
- Reverse flow is detected
Electrical Closure
Bleed valve closes electrically when:
- ENG BLEED pb selected OFF
- ENG FIRE pb pushed
Precooler System
The precooler regulates bleed air temperature using fan air through a modulated fan air valve.
- Target Temperature: Approximately 200°C
- Fan Air Valve: Modulates to maintain target temperature
- No Pressure: Internal spring keeps fan air valve closed
APU Bleed
APU bleed air is available on ground and in flight when APU speed exceeds 95%.
APU BLEED pb ON Commands:
- Opens the APU bleed valve
- Automatically opens crossbleed valve
APU Leak Logic
During an APU leak, the APU bleed valve closes automatically.
Crossbleed Valve
The crossbleed valve interconnects or isolates both engine bleed systems via the crossbleed duct.
Automatic Opening
- When APU bleed is supplying the pneumatic system
Automatic Closure
- Wing, pylon, or APU leak detected (except during engine start)
Leak Detection System
Dual Loop Detection: Leak detection loops installed along wings, pylons, and fuselage ducts.
Valid Leak Indication
A valid leak is detected when:
- Both loops detect overheat, OR
- One loop detects overheat while the other is inoperative
Leak Response Actions
- Wing Leak: Bleed valve on that side closes, FAULT light comes on
- Pylon Leak: Related bleed valve closes, FAULT light illuminates
- APU Leak: APU bleed valve closes automatically
BMC Failure Effects
If one BMC fails, the opposite BMC takes over control but:
- Automatic closure lost for the failed side
- ENG BLEED LEAK caution lost for the failed side
Pneumatic System Uses
Bleed air powers critical systems:
- Engine Start: Via starter motors (pneumatic)
- Air Conditioning Packs: Cooling and cabin pressurization
- Wing Anti-Ice: Thermal ice protection
- Hydraulic Reservoir Pressurization: Maintains hydraulic pressure