The A320 air conditioning system provides cabin and cockpit temperature control, pressurization, and air circulation. The system uses two air conditioning packs fed by engine bleed air to condition outside air. Understanding pack operation, temperature control zones, and pressurization logic is essential for A320 type rating.
System Overview
Key components:
- Two Air Conditioning Packs: Pack 1 and Pack 2, pneumatically operated and electrically controlled
- Three Temperature Zones: Cockpit, forward cabin, aft cabin - independently controlled
- Temperature Range: 18°C to 30°C selectable, default 24°C at 12 o'clock position
- Trim Air Valves: Add hot air upstream of packs for precise zone temperature control
- Mixer Unit: Mixes cold pack air with recirculated cabin air
- Cabin Pressurization: Maximum differential ~8.6 PSI, automatic control via outflow valve
- Ram Air: Emergency ventilation if dual pack failure or smoke
Pack Operation
Each pack cools hot bleed air from engines to desired temperature. Packs operate in AUTO or can be manually selected OFF. Normal operations use both packs. Single pack operation possible but with reduced cooling capacity and passenger load limitations.
Pack Flow Modes
- LO: Approximately 80% of normal flow, used when cabin load is low
- NORM: 100% flow for typical passenger loads
- HI: Maximum airflow, automatically selected in single-pack operation or when APU bleed supplying
📊 Pack Flow Control Valve
Actuation: Pneumatically operated, electrically controlled
No Air Pressure: Valve closes by internal spring force
Automatic Closure: Pack overheat, engine start, or FIRE/DITCHING selected
LO Flow Logic: If LO cannot meet cooling demand, system automatically increases to NORM
💨 Pack Control Logic
Engine Start: Packs automatically shut off during engine start to provide maximum bleed air to starter
Overheat: Packs shut off automatically if pack overheat detected
FIRE/DITCHING: Pack flow control valve closes automatically
APU Bleed + HI Flow: Flow forced to remain in HI when APU bleed supplying (on ground)
Pack Efficiency: Decreases at higher altitude as ram air density drops
Temperature Control
Three independent temperature zones (cockpit, forward cabin, aft cabin) allow different temperatures. Pilots select desired temperature on overhead panel, system maintains automatically using trim air valves.
Temperature Selection
- Range: 18°C to 30°C on AIR COND panel
- Default: 24°C at 12 o'clock selector position
- FAP Authority: Flight attendant panel can adjust ±2.5°C around cockpit-selected reference
Trim Air System
Trim air valves add hot air tapped upstream of both packs (where pneumatic temperature is highest) to achieve precise zone temperature control. Valves are modulated by zone controller based on demand.
- ACSC 1: Controls cockpit zone
- ACSC 2: Controls forward and aft cabin zones
- Hot Air Valve: Closes if both lanes of ACSC 1 fail or if at least two trim air valves fail
⚠️ HOT AIR FAULT
Triggers at: Approximately 88°C duct overheat detected
Extinguishes when: Duct temperature drops below 70°C AND HOT AIR pb is OFF
Action: Press HOT AIR pb OFF to close hot air valve and clear fault when temperature normalizes
Air Distribution and Recirculation
Mixer Unit
The mixer unit blends cold air from packs with recirculated cabin air before distribution to cabin. This improves efficiency by reducing bleed air requirements.
Recirculation Fans
- Quantity: Two fans providing continuous recirculation
- Purpose: Reduce bleed air requirement and improve efficiency
- Automatic Stop: Smoke configuration or ditching selection
Pack Air Cycle Machine (ACM)
- Cooling Source: Ambient ram air entering through heat-exchanger intakes
- Primary Heat Exchanger: Cools bleed air before entering ACM
- Drive: Bleed airflow drives the turbine-compressor assembly
- Water Extractor: Removes humidity from conditioned air before distribution
- Altitude Effect: Pack efficiency decreases at higher altitude as ram air density drops
Pressurization
Cabin altitude automatically controlled by outflow valve. System maintains cabin altitude typically below 8,000 ft. Maximum cabin differential pressure approximately 8.6 PSI. Safety valves prevent over-pressurization.
Pressurization Controllers
- Controllers: Two digital pressurization controllers in active/standby mode
- Schedule Source: FMGC provides cruise level and landing elevation
- Cabin Rate: Pre-programmed based on FMGC vertical profile
- Failure Mode: If both auto controllers fail, manual mode available with direct outflow valve control
Outflow Valve
- Quantity: One single outflow valve
- Actuation: Electrically driven by two independent motors
- Manual Control: Available if automatic controllers fail
Safety Valve
- Protection: Prevents excessive positive or negative differential pressure
- Opens When: Differential pressure exceeds structural limits
Ram Air System
Provides emergency ventilation to cockpit and cabin in case of smoke or dual pack failure.
RAM AIR Operation
- RAM AIR ON + ΔP < 1 PSI: Outflow valve opens to approximately 50% (automatic pressurization)
- Downstream Check Valve: Remains closed whenever ΔP is above 1 PSI
Ram Air Inlet Flap Logic
- Closes During Takeoff: At takeoff power with gear compressed
- Closes On Landing: When speed ≥70 kt
- Reopens After Landing: Approximately 20 seconds after speed falls below 70 kt
- Ground Closure: May close to protect system from contamination at high ΔP
Avionics Ventilation
In smoke configuration, cabin temperature control may be degraded as system prioritizes avionics cooling.