Aircon, Pressurisation and Ventilation (A320)

These notes were written for A320 pilots and engineers who want a quick refresher on the air conditioning, pressurisation and ventilation systems. They follow a question and answer pattern. Your FCOM is always the final reference if anything here seems to conflict with it.

Air conditioning

The pack flow control valve is pneumatically operated and electrically controlled. It closes by itself when a pack overheats, when an engine is starting, or when the fire or ditching pushbutton is operated. It also stays closed when there is no air pressure.

The emergency ram air inlet clears smoke from the cockpit and cabin, or ventilates them if both packs fail. The RAM AIR pushbutton on the AIR COND panel opens it, but only if ditching is not selected. The ram air inlet and outlet flaps for the heat exchangers also close on their own in two cases. On takeoff they close when takeoff power is set and the main gear struts are compressed. On landing they close as soon as the struts compress and the speed is 70 knots or more, then open again 20 seconds after the speed falls below 70 knots. If the pressure difference between cabin and outside is more than 1 psi, the check valve behind the ram air door will not open, even with the door selected open, so no air flows.

The amber HOT AIR FAULT light comes on if the duct overheats (sensed at 88 degrees Celsius) or if the hot air pressure regulating valve disagrees with its selected position. The overheat part resets when the temperature falls below 70 degrees and the crew select OFF. If that valve fails open, nothing happens. If it fails closed, the system loses its optimised regulation and the trim air valves are driven fully closed. Pack 1 then controls the cockpit temperature, and pack 2 controls the cabin temperature for both the forward and aft zones, using a mean of the selected values.

There are three trim air valves, one each for the cockpit, the forward cabin and the aft cabin. Each one adds hot air, taken upstream of the packs, to the mixed air for its zone. If the cockpit trim air valve fails open, the crew see a TRIM AIR SYS FAULT message and the zone trim air valve position turns amber on the ECAM COND page. The three supply zones are the same three: cockpit, forward cabin and aft cabin.

When a pilot turns a temperature selector, a request goes to the zone controller. The cabin knobs set a reference temperature that is fine tuned from the Forward Attendant Panel. The air conditioning system controllers (ACSCs) work out a demand from the selected and actual temperatures. The lowest zone demand goes on to the controllers, which make both packs produce the required outlet temperature.

The ACSC handles pack operation, the pack flow control valve, the hot air pressure regulating valve, the trim air valves, and temperature and flow regulation. Each ACSC controls the temperature of its pack by moving the bypass valve and the ram air inlet flap.

The ECAM BLEED page shows pack outlet temperature, bypass valve position, compressor outlet temperature and pack flow. The ECAM COND page shows two kinds of temperature: the zone temperature for each of the three zones, and the zone duct temperature, which is the temperature of the air in the three main supply ducts. There is no cargo heat for the forward cargo compartment.

The main parts of the system are the air conditioning pack, the air cycle machine, the heat exchanger, the mixing unit, the water separator and the air conditioning system controller.

Pressurisation

The A320 has three pressurisation modes: automatic, semi automatic and manual. In automatic mode the maximum cabin altitude is 8,000 feet.

The system has two cabin pressure controllers (CPCs). One of them drives the outflow valve through its automatic motor. If a pilot suspects the working controller, they can switch to the other by putting the MODE SEL pushbutton to MAN for at least 10 seconds and then back to AUTO. The changeover also happens automatically 70 seconds after each landing, or when the working controller fails. After one CPC failure, the other takes over, a SYS 1 (or 2) FAULT caution appears, and the PRESS page shows on the system display.

With pressurisation in AUTO, pushing open the emergency ram air makes the outflow valve open about 50 percent, as long as the pressure difference is below 1 psi. In MAN mode the outflow valve will not open in that case. In manual mode, the MAN V/S CTL toggle moves the valve: UP opens it, DOWN closes it. The valve moves slowly, so the pilot must hold the toggle until the target rate is reached. Opening the valve raises cabin altitude, and a higher cabin altitude in flight lowers the differential pressure.

Two independent pneumatic safety valves sit on the rear pressure bulkhead above the flotation line. They stop the cabin pressure from rising to 8.6 psi above ambient or falling to 1 psi below ambient. Differential pressure is cabin pressure minus ambient pressure, so a negative value means the cabin pressure is lower than outside.

With the landing elevation selector on AUTO, the controller uses the landing elevation and QNH from the FMGC, and pressure altitude from the ADIRS. If the FMGC data is missing, it uses the captain's baro reference from the ADIRS and the landing elevation selection. With the selector at 1, the landing elevation is plus 1,000 feet.

The residual pressure control unit (RPCU) lets the aircraft depressurise on the ground if residual pressure is abnormal. It opens the outflow valve if the valve is not fully open, both CPCs have failed or manual mode is selected, the aircraft is on the ground, and either all engines are shut down or all ADIRS show airspeed below 100 knots.

When the DITCHING pushbutton is ON, the operating system sends a close signal to the outflow valve, the emergency ram air inlet, and the avionics ventilation inlet and extract valves. The pack flow control valves close too.

Cabin vertical speed appears on three ECAM pages: PRESS, CRUISE, and DOOR (only in flight phases 5, 6 and 7). Phase 5 runs from lift off to 1,500 feet, phase 6 from 1,500 to 800 feet, and phase 7 from 800 feet to touchdown. The display colours work like this:

• Vertical speed pulses green above 1,750 ft/min and resets at 1,650 ft/min.
• Differential pressure pulses green above 1.5 psi in phase 7 and resets at 1 psi.
• Differential pressure turns amber at minus 0.4 psi or below, or at 8.5 psi or above.
• Cabin altitude turns red above 9,550 feet.
• The outflow valve indication turns amber if the valve opens more than 95 percent in flight.

The key parts are the CPC, the RPCU, the outflow valve with its three motors, and the safety valves.

Ventilation

Setting either the BLOWER or the EXTRACT pushbutton to OVRD puts the avionics ventilation in a closed circuit and adds air from the air conditioning system. With BLOWER at OVRD, the blower fan stops and the extract fan keeps running. With EXTRACT at OVRD, the extract fan is controlled straight from the pushbutton and both fans keep running. With both at OVRD, the air conditioning supplies cooling air that is then sent overboard, the blower fan stops and the extract fan runs.

Both go to OVRD when smoke is detected in the avionics ventilation air, which lights the BLOWER and EXTRACT FAULT lights. They also go to OVRD for a SKIN VALVE FAULT with the extract valve affected. These actions send extra closing signals to the inlet and extract valves, and the weather radar picture on both navigation displays may be lost if ventilation is poor.

If the blower fan stops on the ground with engines off, the FAULT light on the BLOWER pushbutton comes on, the ECAM activates and the external horn sounds. Smoke in the avionics duct lights the FAULT lights on both pushbuttons in amber and activates the ECAM. After takeoff power is applied on the ground, the ventilation system is in closed circuit.

Part of the A320 systems series. See also Airbus Tech.