A320 Pressurisation

The A320 pressurisation system keeps the cabin at a breathable and comfortable pressure while the aircraft flies far higher. The packs push conditioned air into the fuselage continuously; the system controls the cabin pressure by regulating how much of it leaves through a single outflow valve. In normal operation the crew have almost nothing to do: the controllers take the landing elevation from the flight management system and build the whole pressure schedule themselves.

Exam questions concern the architecture (how many controllers, motors and valves), the figures of each automatic mode, the switch to the other system and to manual control, the limits, the ditching function and the CAB PRESS page. The general principles are explained in pressurisation principles and control and pressurisation safety valves and warnings; the air supply in A320 air conditioning.

Pressurisation overview

The system has four functions:

• ground function: the outflow valve is kept open on the ground so that no pressure builds up;
• pre-pressurisation: a slight overpressure before take-off avoids a pressure surge at rotation;
• pressurisation in flight: the cabin altitude is controlled to suit each phase of the flight;
• depressurisation: any residual pressure is released after touchdown.

It consists of two cabin pressure controllers (CPCs), one residual pressure control unit (RPCU), one outflow valve whose actuator has three motors, one control panel and two safety valves.

The structural limits are:

CPCs, RPCU and outflow valve

The CPCs receive data from the ADIRS, the FMGC, the engine interface units (EIUs) and the landing gear control and interface units (LGCIUs). The LGCIUs supply the ground or flight status used by the mode logic. In automatic mode one controller is active and the other on standby, linked by a cross-channel link. Control passes automatically to the other system 70 seconds after each landing and whenever the operating system fails. The crew can change over by setting the MODE SEL pushbutton to MAN for at least 10 seconds and then back to AUTO. The MODE SEL amber FAULT light, with its ECAM caution, comes on only when both automatic systems are faulty.

Each controller also contains a backup section for manual operation, supplied by an independent power supply located in the controller 1 position and fitted with its own pressure sensor.

The outflow valve is on the right-hand side of the fuselage, behind the aft cargo compartment and below the flotation line. Its actuator has three independent electric motors, two for automatic operation and one for manual operation, and any one of them can drive the valve.

The residual pressure control unit (RPCU) depressurises the aircraft on the ground if abnormal residual pressure remains. It opens the outflow valve when all of the following are true: the valve is not fully open; both CPCs have failed or manual mode is selected; the aircraft is on the ground; and all engines are shut down or all ADIRS show an airspeed below 100 kt. The concern is the doors: on each passenger door a red cabin pressure light flashes when both engines are stopped, the slides are disarmed and the cabin pressure is more than 2.5 hPa above ambient.

Automatic pressurisation modes

With the LDG ELEV selector at AUTO, the controllers use FMGS data, including the landing field elevation, to build an optimised pressure schedule. They then run through a sequence of modes:

The system is optimised for high airfields: on the ground, in the climb and in the descent it supports an airfield pressure altitude of up to 14,100 ft, although the cruise cabin altitude remains limited to 8,000 ft. If FMGC data are not available, the controller uses the captain's barometric reference from the ADIRS and the landing elevation selected on the panel.

Landing elevation and manual mode

The LDG ELEV selector is normally at AUTO. Turned away from AUTO, it sets the landing elevation manually anywhere from −2,000 ft to +14,000 ft. The scale on the knob is only indicative, so the crew set the value by reading it on the ECAM. The green MAN LDG ELEV memo appears whenever the selector is not at AUTO, and the CAB PRESS page then shows the landing elevation as MAN instead of AUTO. When FMGS data are unavailable, selecting the landing field elevation is all the crew need to do.

Manual mode is selected with the MODE SEL pushbutton at MAN. The crew then control the cabin altitude through the manual motor of the outflow valve, using the MAN V/S CTL toggle switch. The switch is spring-loaded to neutral: holding it UP drives the valve towards open, so the cabin climbs, and DN drives it towards closed. To obtain a precise cabin vertical speed, only short inputs should be applied.

Manual control has its own characteristics:

• the outflow valve moves slowly, and the ECAM may take up to 5 seconds to show a change in its position;
• on switching from AUTO to MAN, the cabin altitude indication may change slightly, because the backup pressure sensor has a lower resolution;
• the outflow valve does not open automatically at touchdown, so the crew must open it, although the RPCU may still open it on the ground under its own logic.

Safety valves and unpressurised areas

Two independent pneumatic safety valves are installed on the rear pressure bulkhead, above the flotation line. They prevent the cabin pressure from rising more than 8.6 psi above ambient or falling more than 1 psi below it. Because the differential pressure measurement is less accurate in manual mode, and because the differential falls as soon as a safety valve opens, the ECAM may show a safety valve open at an indicated differential pressure anywhere between 8.9 and 9.7 psi.

Not all of the fuselage is pressurised. The A320 has four unpressurised areas: the nose gear bay, the air conditioning compartment, the main gear bay and the tail cone, which houses the APU. The cockpit, cabin, avionics compartment and cargo compartments lie within the pressure hull.

Ditching and the flotation line

The flotation line is the waterline the aircraft would float at after a ditching. The outflow valve lies below it and the safety valves above it. The guarded DITCHING pushbutton closes the valves below the flotation line:

• the outflow valve;
• the emergency ram air inlet;
• the avionics ventilation inlet and extract valves;
• the pack flow control valves;
• the forward cargo outlet isolation valve.

The cabin recirculation fans are not affected. If the outflow valve is under manual control when DITCHING is selected, it is not closed automatically and the crew must close it with the manual motor. On the ground, with a low-pressure air conditioning cart connected and all doors closed, selecting DITCHING lets a differential pressure build up in the cabin, because the openings that would vent it are shut.

The DITCHING selection also overrides the RAM AIR pushbutton: the emergency ram air inlet stays closed. Without ditching, selecting RAM AIR with a differential pressure below 1 psi makes the outflow valve open about 50 % under automatic control, but not under manual control.

CAB PRESS page and warnings

The ECAM CAB PRESS page shows the landing elevation (AUTO or MAN), the differential pressure, the cabin vertical speed and cabin altitude, the outflow valve and safety valve positions, the active system and the packs. Its thresholds are:

The page also carries the avionics ventilation indications (VENT, BLOWER, EXTRACT, INLET, OUTLET), described in A320 ventilation and cargo heating. The DOOR/OXY page shows the cabin vertical speed during flight phases 5, 6 and 7; it pulses green when its absolute value exceeds 1,800 ft/min for more than 5.5 seconds and stops below 1,600 ft/min.

Above the CAB PRESS thresholds, other systems respond to cabin altitude directly. If it exceeds 11,300 ft (± 350 ft), the cabin signs come on whatever their switch positions, except the NO PORTABLE ELEC DEVICE signs, and so do the cabin lights, depending on the cabin system programming. With the MASK MAN ON pushbutton at AUTO and HI ALT LANDING off, the passenger oxygen mask doors open automatically above 14,000 ft cabin altitude. The handling of a loss of pressure is covered in decompression.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.