AIRCRAFT PRESSURIZATION

 

The pressurization system is made up of two identical, independent, lautomatic systems (system 1 or system 2) that control the cabin altitude. The overall system consists of two cabin pressure controllers (CPCs), one outflow valve, two safety valves and a control panel. The outflow valve actuator incorporates three electric motors; two for automatic control and one for manual control.

In normal operation the system is operated in automatic control, however it can also be operated in semi-automatic or manual control. The CPCs receive the landing elevation, QNH and pressure schedule from the FMGS, and the pressure altitude from the ADIRS. The engine Interface Unit (EIU) supplies the throttle lever position and the LGCIU sends the flight/ground signal.

In case of FMGC failure, the crew sets the landing elevation manually by pulling the LDG ELEV knob out of the auto detent, rotating it to the desired landing elevation and the system then uses its own internal pressure schedule (semi-automatic control). In this case, the controller uses the Captain’s Baro reference from the ADIRS.

In manual mode the crew controls the cabin altitude by use of the switch control on the CABIN PRESS panel, which in turn operates the manually controlled electric motor on the outflow valve actuator.

The safety valves prevent cabin pressure from exceeding 8.6 psi or going below 1 psi below ambient pressure.

AUTOMATIC PRESSURE CONTROL SEQUENCE 

Ground 

Before takeoff and 55 seconds after landing, the outflow valve fully opens to ensure that there is no residual cabin pressure. At touchdown, any remaining cabin pressure is released at a cabin vertical speed of 500 ft/min. An automatic transfer of systems occurs after each landing.

Takeoff 

To avoid a pressure surge or ‘bump’ at rotation, the cabin is pre-pressurized when the thrust levers are advanced, at a rate of 400 ft/min until ΔP reaches 0.1 psi. At lift off the CPC initiates the climb phase.

Cruise 

The CPC maintains cabin altitude at the level-off value or the landing elevation, whichever is higher.

Descent 

During descent the controller maintains a cabin rate of descent so that the cabin pressure equals the landing field pressure, just before landing. The maximum rate of descent is 750 ft/min.

Abort 

If the aircraft does not climb after takeoff or aborts the mission, the abort mode prevents the cabin altitude from climbing. Cabin pressure is set to the takeoff altitude + 1 psi.

System transfer 

During normal operations one system is in active mode and one is in standby. If one fails the other will automatically take over. If the crew suspect a system to be malfunctioning, they can transfer over to the other one, by selecting the MODE SEL pb to MAN for 10 seconds and then returning to AUTO.

MANUAL PRESSURE CONTROL 

If both systems fail, the crew can control the pressurization manually by the following procedure: 

• Press the MODE SEL pb on the CABIN PRESS panel. 

• Push the MAN V/S CTL switch UP or DN to increase or decrease cabin altitude.

Note: Due to the slow operation of the outflow valve in manual mode, and the limited resolution of the outflow valves’ position on the ECAM, the visual ECAM indication of an outflow valve position change, can take up to 5 seconds. It is therefore prudent to make small adjustments. DITCHING

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