The cabin pressure controller controls the pressure in the cabin in relation with aircraft altitude, and a manual control is possible using the overhead panel:

A320 Air Conditioning and Cabin Pressure panels A320 Air Conditioning and Cabin Pressure panels on an Airbus A320 (source: Olivier Cleynen on Wikimedia)

B737 Pressurization panel B737 NG
Pressurization panel on a Boeing 737 NG (source: B737.org.uk)

Overall how do the controller and the related equipment work in a commercial airliner, e.g:

  • What is the source used for pressure reference?
  • How outflow and relief valves and packs are related to the controller?
  • When and how is used the manual control of the pressure?

1 Answer 1


The System you are referring to is described in ATA chapter 21. I will report here an example for A320

An overview of the system is reported below (from the ATA21 report):

The schematic of an A320 pressurization system

What is the source of pressure reference?

In flight the outside pressure signal can be come from either:

  • The Air Data Reference System (ADIRS), for the pressure altitude.
  • The Flight Management and Guidance Computer (FMGC), for the landing elevation.
    • If FMGC not available, uses the landing elevation from the ADIRS and the landing elevation

How the different valves (outflow, relief) and the packs are related to the CPC?

There are two independent safety valves controlled by two independent Cabin Pressure Controllers (CPC) and actuated by three independent motors. The pressure in the cabin is controlled only with these valves, while the temperature of the cabin is controlled by mixing the air from the packs with the one of the cabin. In particular the valves are opened and closed to keep the desired pressure profile according to the logic dictated by the different flight phase. Pressurization Profile.

Different constraint drive the controller:

  • Cabin pressure at cruise altitudes is limited to an equivalent ISA pressure of 8000ft, due to structural fuselage requirements. By providing higher pressures levels (equivalent to lower ISA equivalent altitudes, e.g.: sea level) the fuselage would overstrain. A (very) simplified example of this is a balloon in a vacuum chamber.
  • Maximum descent rate is kept under 750 ft/min for passenger comfort. Otherwise your internal ear starts hurting.
  • At take-off on ground the cabin is de-pressurized till -0.1 psi with a max rate of 400ft/min to avoid pressure surge at rotation

When and how is used the manual control of the pressure?

Manual control of the pressure is used when both the CBC controllers fail or when the crew uses the cabin press control panel, that you showed. In case of aircraft ditching the inlets are closed as well with a ditching button (first image, right in the CABIN PRESS panel)

  • 1
    $\begingroup$ Interesting answer. I discovered the existence of ATA chapters, thanks a lot! $\endgroup$
    – mins
    Commented Mar 23, 2016 at 14:22
  • $\begingroup$ This answer deserves a little more work. Obviously the cabin pressure goes much higher than the pressure of the ambient atmosphere at 8000'. Also obviously a hgher cabin pressure altitude would decrease not increase the strain on the aircraft. Did you mean to say that the difference between cabin pressure and outside pressure is kept to 8.06 psi maximum, or the cabin would explode like balloon? $\endgroup$ Commented Nov 22, 2019 at 20:22
  • $\begingroup$ Also, can you please say more about this business of pressurizing to below field altitude before takeoff, "to avoid a pressure surge at rotation"? And is the same thing happening before landing? I don't get it, seems to just inflict unnecessary pressure changes on the passengers' ears. Also did you mean to say before take-off on ground the cabin is pressurized (not de-pressurized) till -0.1 psi with a max rate of 400ft/min to avoid pressure surge at rotation? And can you please explain what would cause a pressure surge at rotation? $\endgroup$ Commented Nov 22, 2019 at 20:24

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