A320 overhead panel - MODE SEL selected to MAN. From this moment, the cabin is yours.

By Paul Toensing  |  Custom GPT Solutions  |  May 2026  |  ~8 min read

The Automation You Have Never Flown

There is a category of system on the A320 that pilots interact with on every flight without actually flying. The Cabin Pressure Control system is one of them. In AUTO, the pressurization system uses FMGS data to construct its pressure schedule. From pushback until 55 seconds after touchdown, the Cabin Pressure Controllers run the cabin without a single conscious input from anyone in the cockpit. They are the quietest, most consequential automation on the airplane.

Which is why, when the ECAM lights up with CAB PR SYS 1+2 FAULT, pilots who can recite the procedure from memory still freeze for a beat. Both Cabin Pressure Controllers have failed. The cabin is yours. And the dirty secret is that you have never actually flown one before.

This piece is the architectural companion to our AEO page on specific A320 pressurization procedure questions and a deeper extension of the published LinkedIn article. It walks the parts of the system that rarely make it into informal pilot conversation - the two-controller architecture, the three-motor actuator, the safety net with four conditions, the door-opening procedure that gets misremembered, and the dispatch envelope most pilots get wrong.

The Two Computers That Take Turns

The A320 has two identical, independent digital Cabin Pressure Controllers. Only one operates at a time. An automatic transfer occurs 70 seconds after each landing, or if the operating system fails. The standby CPC is not a passive spare - it runs in parallel, continuously, watching the same world the active controller sees.

And here is the architectural fact behind QRH OEB61: the standby CPC can detect abnormal cabin altitude and trigger the CAB PR EXCESS CAB ALT alert even while the active CPC is displaying an erroneous cabin altitude within the normal range on the CAB PRESS SD page.

Read that twice. The alert is real. The display is wrong. The procedure does not give you the option to wait until the SD page agrees. The standby has not been wasting its career, and when it speaks up, it is the only honest voice in the room.

Source: FCOM DSC-21-20-30, p.1/4, Rev 25 NOV 24 + DSC-21-20-20, p.1/2, Rev 22 MAR 16

The Valve With Three Motors

The outflow valve actuator contains the drives of two automatic motors and one manual motor. Either automatic motor operates the valve in automatic mode. The manual motor operates it in manual mode. Normally, one CPC drives the outflow valve through its associated automatic motor.

When you select MODE SEL to MAN, the change is binary and complete. Power to the AUTO motors is cut. The MAN motor is enabled. There is no soft handoff, no gradual transition. One moment the AUTO system is flying the cabin. The next moment, you are.

The selector is spring-loaded to neutral. UP commands the valve toward open - increasing cabin altitude. DN toward closed - decreasing cabin altitude. To target a precise cabin vertical speed, only short inputs should be applied. The selector rewards patience. It punishes anything else.

In automatic pressure control mode, at touchdown any remaining cabin pressure is released at a cabin vertical speed of 500 ft/min. Fifty-five seconds after landing, the outflow valve fully opens to ensure there is no residual cabin pressure. In MAN mode, none of that happens automatically. You are the schedule.

Source: FCOM DSC-21-20-20, p.1/2, Rev 22 MAR 16 + DSC-21-20-30, p.1/4, Rev 25 NOV 24

Ten Seconds Versus Five Seconds

The procedural warning in CAB PR SYS 1+2 FAULT is unambiguous: in manual mode, it may take 10 seconds before the crew notices a change in outflow valve position. The procedure tells you specifically to use cabin V/S to confirm outflow valve operation.

Separately, the system description notes that the visual ECAM indication of an OFV position change can take up to 5 seconds.

Two different facts. The 10-second warning is operational - it tells you what to expect when you command the valve. The 5-second figure is a display latency note from the system description. Pilots who conflate the two end up watching the OFV position indication when they should be watching cabin V/S, which the manual tells you specifically to use as the operational confirmation.

Use cabin V/S to confirm operation. Apply short inputs. The cabin will catch up.

Source: FCOM PRO-ABN-CAB_PR, p.12/14, Rev 25 NOV 24 + DSC-21-20-30, p.3/4, Rev 25 NOV 24

The Door Procedure That Gets Misremembered

Here is the trap that catches even experienced pilots in oral exams. There are two different procedures around opening cabin doors after a pressurization event, and they are not the same procedure.

The FCOM Abnormal Procedure

The FCOM CAB PR SYS 1+2 FAULT procedure says only this about doors: check that delta-P is zero before opening the doors. That is the standard post-fault sequence. The crew confirms the differential pressure has been driven to zero through the manual MAN V/S CTL inputs during approach and landing - then doors can be opened. This is the procedure that applies after every CAB PR SYS 1+2 FAULT landing.

The MEL MO-52-10 52-10-10A Procedure

MEL MO-52-10 52-10-10A is a different procedure. It applies when the Door Residual Differential Pressure Detection equipment is itself inoperative under MEL dispatch. In that case the sequence is more involved: PACK 1 OFF, PACK 2 OFF; check delta-P = 0.0 PSI and outflow valve fully open on the SD page; then open one cockpit sliding window. If the flight crew cannot open one cockpit sliding window, residual pressure remains and the cabin doors must not be opened.

The two procedures look similar in spirit but they apply to different situations. The FCOM procedure applies after every CAB PR SYS 1+2 FAULT landing - it is the abnormal procedure. The MEL procedure applies when the Door Residual Differential Pressure Detection equipment is deferred under dispatch - it is a workaround for missing safety hardware. Confusing the two is the kind of detail that separates a candidate who has read the manuals carefully from one who has read summaries.

Source: FCOM PRO-ABN-CAB_PR, p.13/14, Rev 25 NOV 24 + MEL MI-52-10/MO-52-10, 52-10-10A, Rev 25 FEB 25

The Safety Net With Four Conditions

The RPCU - the Residual Pressure Control Unit - automatically depressurizes the aircraft in case of abnormal residual pressure on the ground. It opens the outflow valve, but only when all four activation conditions are met simultaneously:

1. The outflow valve is not fully open.
2. Both CPCs are failed, or MAN mode is selected.
3. The aircraft is on the ground.
4. All engines are shut down, or all ADIRS indicate airspeed below 100 kt.

Three of those conditions describe the state of the aircraft. The fourth - all engines shut down or all ADIRS below 100 kt - is the timing variable. The RPCU does not intervene the moment you touch down. It waits.

This is why the RPCU is the safety net, not the plan. By the time it activates, you should have already executed the procedure: MAN V/S CTL FULL UP on final, delta-P = 0 confirmed at the airfield pressure altitude + 2,500 ft gate, the cabin already depressurized. The RPCU is the architectural backup for the version of the crew that did not do any of that. Treat it as nice-to-have. Never as door-opening clearance.

Source: FCOM DSC-21-20-20, p.2/2, Rev 22 MAR 16

The Dispatch Reality Most Pilots Get Wrong

Under MEL 21-31-01C, the aircraft can dispatch with both automatic CPCs inoperative under Category C, provided two conditions are met: the manual cabin pressure control system is operative, and the flight is not pressurized.

Read that carefully. The aircraft can fly. It cannot fly pressurized. The applicable procedure is FCOM PRO-NOR-SUP-MISC-E Flight Without Cabin Pressurization, which imposes altitude restrictions, oxygen requirements, and operational constraints. It is the legal pathway out of an outstation with a dual CPC failure, but it is not the kind of leg you brief casually.

Most pilots assume both-CPC dispatch is prohibited. It is not. But it is also not casual ops.

Single-CPC failures have their own item-specific limits in MEL 21-31-01A/B. Dispatch with Automatic CPC 1 inoperative is prohibited for ETOPS operations. The CPC 2 item is different. The takeaway is not a generalized ETOPS rule but a reminder that each MEL item carries its own restrictions and must be read individually.

Source: MEL MI-21-31, 21-31-01A/B/C, p.1/4-2/4, Rev 18 NOV 20

What QRH OEB61 Actually Says

QRH OEB61 is unambiguous: apply the CAB PR EXCESS CAB ALT ECAM procedure even when the cabin altitude reads normal on the CAB PRESS SD page. The reason is the two-controller architecture - the active CPC may be displaying an erroneous normal cabin altitude, while the standby CPC has detected the abnormal condition and triggered the alert.

The standby CPC's cabin altitude is not displayed anywhere on the SD page. The warning is the only signal you get. Trust it, even when the SD page appears stable. The procedure does not include an option to wait for confirmation.

Source: QRH OEBPROC-61, p.61.01A-61.02A, Rev 25 APR 24

The Questions the Oral Examiner Is Actually Asking

If someone asked you right now to walk through the four RPCU activation conditions in order, to distinguish the FCOM abnormal door procedure from the MEL 52-10-10A procedure and explain exactly when each applies, or to state whether MEL 21-31-01C permits pressurized flight - could you answer without reaching for the manual?

The pressurization system is Subject 5 for a reason. It is genuinely complex, the automation runs so quietly that most pilots never develop real manual intuition for it, and the procedural distinctions are precise enough that half-right answers reveal themselves immediately under examiner questioning.

For the discrete procedural answers - each Q&A anchored to current HKA FCOM, QRH, and MEL - see the AEO page on A320 Pressurization MAN Mode.