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When you web-search an answer for how airliners maintain cabin air breathability, the typical answer you get is that bleed air is the key: new air comes in from the outside and gets mixed with old air.

But these days, some airliners are bleedless, such as Boeing's 787.

So, what maintains air breathability/quality in the cabin? Do they use oxygen tanks and some kind of CO₂ capture mechanism? What about other kinds of air treatment, e.g. for other gases, organic material etc?

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    $\begingroup$ This is an airplane, not a spaceship. There's still a lot of oxygen outside the cabin that you can just pump in. $\endgroup$
    – user3528438
    Commented Jul 22, 2023 at 0:23
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    $\begingroup$ @user3528438 Well, it's not "just" pump in. It is a pressurized body. $\endgroup$
    – einpoklum
    Commented Jul 22, 2023 at 8:40
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    $\begingroup$ Bleed-air must be cooled with cold air from the outside. So there are already cold air (ram air) inlets used by the heat exchangers with bleed ECS, even if this air doesn't reach the pressure vessel. Here is a traditional pack with its prominent ram air inlet. (Source). $\endgroup$
    – mins
    Commented Jul 22, 2023 at 14:29
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    $\begingroup$ Here (pdf) you'll find a nice Boeing document about the bleedless system used on the B787. $\endgroup$
    – sophit
    Commented Jul 22, 2023 at 15:25

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Bleedless aircraft still get air from outside. They just have dedicated compressors to compress outside air for cabin use, rather than relying on bleed air from the engines.

The air compressors have outside air ducted directly to their inputs and compress the air to the proper cabin pressure. It works identically to bleed air otherwise- fresh outside air is brought in through the compressors and stale inside air leaks out or flows out through dedicated outflow valves.

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    $\begingroup$ This system along with thermal electric anti-icing forced Boeing to go with very large L-ion batteries, leading to the fires, and the need to build a L-ion-fire-proof enclosure after grounding the fleet. On the C-series/A-220, there was considerable thought given to using them to save weight, there being significant electrical services (although not like the 787), but they, perhaps wisely, decided to stick with nicads. $\endgroup$
    – John K
    Commented Jul 22, 2023 at 1:35
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    $\begingroup$ @einsupportsModeratorStrike See the image in this answer. $\endgroup$
    – Bianfable
    Commented Jul 22, 2023 at 11:47
  • $\begingroup$ @Bianfable: Is that also used for exhausting air? $\endgroup$
    – einpoklum
    Commented Jul 22, 2023 at 12:38
  • $\begingroup$ @einsupportsModeratorStrike No, the ram air has specific ram air outlets a bit further aft. The cabin air is exhausted through the outflow valve(s). They are typically located towards the aft of the cabin (sometimes there is one further forwards, but not sure for the 787). $\endgroup$
    – Bianfable
    Commented Jul 22, 2023 at 12:45
  • $\begingroup$ @JohnK: "This system along with thermal electric anti-icing forced Boeing to go with very large L-ion batteries" that's the concept of MEA, more electric aircraft, the generators power also increased from 600 kVA (A380) to 1000 kVA (B787). "leading to the fires", Airbus continued with Ni-Cd units for the A350 after Boeing's problems, but that's only temporary. More in Aircraft batteries: current trend towards more electric aircraft $\endgroup$
    – mins
    Commented Jul 22, 2023 at 22:27
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tl;dr: Through air intake holes using compression.

For example, here they are on the fuselage of a Boeing 787, below the forward edge of the wing:

enter image description here

  • Bottom intake air gets compressed (and thus heated)
  • Top intake air is used to cool the compressed air (and is called "ram air"; it never goes into the cabin)
  • Cooled, compressed air is then released into the cabin (somehow)

Further reading:

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