When waiting to depart Edinburgh today I noticed a square hole (looked to be something that can be opened and closed) on the rear of the right side of the aircraft.

Never seen it before and no idea what it's purpose is. Anyone know?

The opening is below and to the right of the rear door on the right hand side of the aircraft A320 (image source)

  • $\begingroup$ I was unable to upload the image directly. If someone is able to edit the post and upload the image from the link that would be great $\endgroup$ Jul 28, 2019 at 15:27
  • $\begingroup$ To save others a web search, G-UZHG seems to be an A320-251N(SL). $\endgroup$
    – user
    Jul 28, 2019 at 16:16
  • $\begingroup$ Can you add a circle around the spot by any chance? $\endgroup$
    – dalearn
    Jul 28, 2019 at 18:00
  • 2
    $\begingroup$ It serves as a place for which to insert a round peg $\endgroup$ Jul 28, 2019 at 18:09
  • $\begingroup$ @quietflyer I got a chuckle out of that. $\endgroup$
    – John K
    Jul 28, 2019 at 18:45

1 Answer 1


Cabin pressurization outflow valve.

Cabin pressure in an airliner is maintained by bleeding air from the engine compressors into the cabin, after conditioning (usually cooling and humidifying) it. Current designs are beginning to use separate air compressors for this.

In either case, the cabin pressure is regulated by controlling the difference between inflow, provided by the bleed or compressor, and outflow, which is a simple motor-controlled valve.

Generally the inflow is set to maximum and the outflow is varied, as this provides the maximum fresh air flow for the passengers.

See also:

How does the outflow valve work on a cabin pressurization system?

How does the cabin pressure controller work in a large commercial airliner?

How does the cabin pressure controller work in a large commercial airliner?


Can an aircraft re-pressurize at high altitude?

  • $\begingroup$ That's not an outflow valve. Outflow valves are normally on a bulkhead in the interior pressure hull, and dump into an unpressurized area like the tail cone or unpressurized equipment bay. That's is more likely to be a dump valve for ground operation to ensure the fuse stays unpressurized with the packs running. $\endgroup$
    – John K
    Jul 28, 2019 at 18:50
  • 1
    $\begingroup$ @JohnK: Most jet-liners have an outflow valve like that shown in the photo. The location may vary, for example on the MD-11 it's on the forward fuselage (port side), but it's the same shape and function. A smaller jet like an Embraer outflows aft of the bulkhead to cool the aft electronics bay. $\endgroup$
    – user14897
    Jul 28, 2019 at 21:31
  • $\begingroup$ Yes but that little door is not an outflow valve. Outflow valves are regulating poppet valves. Most airliners have some kind of way to bypass the normal pressurization outflow valves dump air overboard on the ground and I assume that's what that is. $\endgroup$
    – John K
    Jul 28, 2019 at 22:37

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