The document describes the air conditioning system on an Airbus A320, which uses two air conditioning packs to cool bleed air and distribute it to different zones of the aircraft. Hot air can be added to the zones via trim air valves to adjust the temperature. The cargo area also has a heating system that operates on similar principles to the main air conditioning system.

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The A320 is equipped with two
air conditioning packs located
in the wing root area forward
of the landing gear bay.
Let’s look at how a pack
works.

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HOT BLEED AIR
Pack Flow
Control Valve
Hot bleed air enters the pack via a pack flow control valve.
The purpose of this valve is to adjust the flow rate
through the pack.

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Cool Conditioned Air
The air then passes through several stages within the pack that
progressively cool the air to provide a conditioned air output. In
fact, sub-zero temperatures can be achieved.
HOT BLEED AIR

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Cool Conditioned Air
Two of the stages are heat exchangers that use a flow of ambient air
over them to cool the hot bleed air. This flow of air enters via a pack
inlet scoop and exits via an outlet duct.
HOT BLEED AIR
Pack Inlet Scoop Outlet Duct

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By-Pass
Valve
HOT BLEED AIR
So that the output temperature of the
pack can be adjusted, a by-pass
valve is fitted.
This valve allows warmer air to be
mixed with the cold air.
Note: The by-pass valve provides for greater fuel efficiency. Since the pack,
in many circumstances, can be by-passed, less bleed air is needed from the
engines. Thus, less thrust is needed, and therefore, less fuel.

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Pack Flow Control Valve
The pack flow control valve, and
the by-pass valve, are regulated
by a pack controller to vary the
flow rate and temperature output.
Pack
Controller
By-Pass
Valve

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The pack controller also controls the inlet scoop and outlet duct to vary the amount of air
passing over the heat exchangers.
Note: To avoid ingestion of foreign matter the inlet scoop and outlet duct close
temporarily during take off and landing.
Pack Inlet Scoop Outlet Duct
Pack
Controller

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Pack 1 Pack 2
The two air conditioning packs operate automatically
and independently to provide cool conditioned air.
HOT BLEED AIR
Cool Conditioned Air
Air Conditioning Packs

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Mixing Unit
Pack 1 Pack 2
The conditioned air from the packs is then fed to a
mixing unit.

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Ram Air
MIXING UNIT
Pack 1 Pack 2
In case of failure of both packs, ram air is provided via a ram air
valve. We will look at the use of ram air in the abnormal operation
module.
Ram Air
Valve

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Various pack parameters are monitored by ECAM.
They are:
- Pack flow
- Compressor outlet temperature
- By-pass valve position
- Pack outlet temperature
o
C
5
o
C
15
PACK OUTLET
TEMP
PACK OUTLET
TEMP
BY-PASS VALVE
POSITION
BY-PASS VALVE
POSITION
o
C
120
o
C
115
COMPRESSOR
OUTLET TEMP
COMPRESSOR
OUTLET TEMP
PACK FLOW
PACK FLOW
MIXING UNIT
Pack 1 Pack 2
Ram Air

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o
C
5
o
C
15
PACK OUTLET
TEMP
PACK OUTLET
TEMP
BYPASS VALVE
POSITION
BYPASS VALVE
POSITION
o
C
120
o
C
115
COMPRESSOR
OUTLET TEMP
COMPRESSOR
OUTLET TEMP
PACK FLOW
PACK FLOW
Ram Air
MIXING UNIT
Pack 1 Pack 2

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[][][][][][] [][]
These parameters, along with the valve positions, are
displayed on the upper part of the ECAM BLEED page.

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ECAM BLEED Page
Having discussed the air conditioning packs we will now
look at how temperature and flow regulation is achieved
throughout the aircraft.

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Three Temperature Zones
Mixing Unit
The three outputs from the mixing unit feed three separate
aircraft zones, cockpit, forward cabin, and aft cabin.
Let’s look at how the temperature of the zones is managed.
Pack
1
Pack
2

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Mixing Unit
A zone control computer monitors the temperatures of the three
zones and sends signals to the pack controllers to set the air
temperature delivered by the packs.
Pack
1
Pack
2
Pack
Controller
Pack
Controller
Zone Control
Computer

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Mixing Unit
Normally cold air is delivered by the packs and is then
sent to the three zones.
Pack
1
Pack
2
Cold Air

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Hot
Bleed Air
HOT
AIR
Trim Air Valves
Mixing Unit
Since the different areas of the aircraft may require different
amounts of cooling, or heating, hot bleed air can be added via
trim air valves to achieve the demanded temperature for a zone.
Pack
1
Pack
2

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Hot
Bleed Air
HOT
AIR
Mixing Unit
Pack
1
Pack
2

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Hot
Bleed Air
HOT
AIR
Mixing Unit
Pack
1
Pack
2

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In the example shown:
• no hot air is being
added to the cockpit
zone
• some hot air is being
added to the forward
cabin zone
• a lot of hot air is
being added to the
aft cabin zone
Hot
Bleed Air
HOT
AIR
Mixing Unit
Pack
1
Pack
2

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Hot Air
Valve
The trim air valves are supplied via a hot air valve. The
purpose of this valve is to regulate the pressure of the hot
air supplied to the trim system and to act as a shut off.
Hot
Bleed Air
HOT
AIR
Mixing Unit
Pack
1
Pack
2

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The hot bleed air is supplied to the hot air valve from just
downstream of the pack flow valves. This means that if the pack
flow valves are closed there is no air supplied to the trim system.
The hot air valve and the trim air valves will automatically close.
HOT
AIR
Mixing Unit
Pack
1
Pack
2

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HOT
AIR
Mixing Unit
Pack
1
Pack
2
Zone Control
Computer
The trim air valves and the hot air valve are controlled by
the zone control computer.

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Zone Duct
Temperature
HOT
AIR
Mixing Unit
Pack
1
Pack
2
Zone duct temperature, which is the temperature of
the air entering a zone,
72 77 84

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Zone
Temperature
and the actual zone temperature are monitored by the zone
control computer and presented on ECAM.
HOT
AIR
220
0
72 84
77
74 70 68

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ECAM Air Conditioning Page
The ECAM COND page contains zone temperature
and trim air system indications.

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ECAM CRUISE Page
The ECAM CRUISE page also contains zone
temperature indications.

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On the overhead panel there is an AIR COND panel which allows
pilot control of the air conditioning system.
Let’s look at the controls on this panel in a little more detail.

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The PACK FLOW switches control their
associated pack flow control valves.
OFF
[][][][][][] [][]
OFF

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OFF
In the example shown PACK 1 is selected off and is indicating
closed, while PACK 2 is selected on and is indicating open.
OFF

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The PACK FLOW selector permits selection of desired pack flow.

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[][][][][][] [][]
The guarded RAM AIR switch controls the RAM AIR valve.
In the example shown the RAM AIR valve is indicating closed.
Use of the RAM AIR switch is restricted to abnormal
conditions that will be discussed later.

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The HOT AIR switch is associated with the HOT AIR valve. There is a
HOT AIR valve indication on the ECAM COND page.
In the example shown the valve is indicating open.
Now let’s look at temperature control.
HOT AIR Valve

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Zone Temperature Selectors
The zone temperature
selectors are used to
adjust the required
temperature for an
associated zone.

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HOT
AIR
Mixing Unit
Pack
1
Pack
2 Cabin
Fan
Cabin
Fan
Two cabin fans are fitted to reduce the bleed air requirement, and
therefore save fuel. These fans establish a recirculation flow of air from
the three aircraft zones to the mixing unit. In normal operation there are
no ECAM indications associated with the cabin fans.

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On the overhead panel there is a CAB FANS switch on the
VENTILATION panel to control the cabin fans. The CAB FANS switch
can be used to switch the fans off in response to ECAM procedure.

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AFT
Inlet Isolation
Valve
Cabin
Ambient Air
The conditioning of the air in the cargo
compartment is fully automatic. Ambient air from
the cabin area enters the cargo compartment via
an inlet isolation valve.

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AFT
Extractor fan
Outlet Isolation
Valve
Overboard
Cabin
Ambient Air
The air is removed from the compartment
either by an extractor fan, or by differential
pressure.
The air is discharged overboard via an outlet
isolation valve.

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AFT
Cargo
Ventilation
Controller
Overboard
Cabin
Ambient Air
The operation of the isolation valve and the
extraction fan is controlled automatically by a
cargo ventilation controller.

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To provide heating of the cargo compartment, hot
bleed air is supplied via a trim air valve. The
operation of the cargo trim air system is very
similar to the trim air system for the air
conditioning system.
AFT
Cabin
Ambient Air HOT BLEED AIR
Trim Air
Valve

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A cargo heating controller controls the trim air valve
position to regulate the temperature of the compartment.
The information is presented on the ECAM COND page.
AFT
Cabin
Ambient Air HOT BLEED AIR
CARGO HEATING
CONTROLLER

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The cargo heating system is an extension of the air
conditioning system. Hot air is routed via an independent
HOT AIR valve for the aft cargo hold.

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The system has a trim air valve and two isolation valves which
close immediately should smoke be sensed, thus turning the
aft cargo bin into a Class D compartment.
•Isolation Valves
Trim Air Valve

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The trim air valve is identical in operation to the trim air
valves for cabin conditioning. It is used to add or subtract
hot air.

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Duct temperature, and cargo compartment temperatures are
shown on the ECAM page.

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On the overhead panel there is a CARGO HEAT panel that
contains the controls associated with cargo heating and
conditioning.

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This is the ISOLATION VALVE switch. This switch
normally remains in the OPEN, or “lights out”
position.

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The temperature selector sends demand signals to the CARGO
HEAT controller. The CARGO HEAT controller then moves the
trim air valve to adjust the temperature of the air entering the
compartment.

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The HOT AIR switch controls the aft cargo compartment HOT
AIR valve via the aft cargo heating controller. This switch also
normally remains in the “lights out” auto position.
Cargo Heating
Controller

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For normal operation of the cargo conditioning system the only pilot inputs required
are:
• to confirm that the switches are in their normal “lights out” position
• to set the required temperatures
Note that the mid position of the temperature selector is approximately 60 degrees F.

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LIST OF SUBJECTS
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Switches & Indicators
Cargo Conditioning
Air Conditioning System