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The air inside an airliner is very dry which causes all sorts of passenger discomfort such as dry eyes, dry skin and in some cases headaches.

Why is the air so dry and what do or can modern designs do to improve the air humidty on board an airliner?

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    $\begingroup$ Maybe because it basically is ambient air (first heated up by the being squeezed into the engine and then again cooled down by the aircon packs) and ambient air at 30,000 ft tends to be dry. But I bet you're asking why the aircon packs don't humidify it any further. $\endgroup$ – PerlDuck Oct 30 at 9:31
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    $\begingroup$ Related: Can an airliner provide ground level cabin pressure, temperature, humidity etc? $\endgroup$ – fooot Oct 30 at 14:23
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    $\begingroup$ Related: Why does cabin air have to be dehumidified? $\endgroup$ – fooot Oct 30 at 14:24
  • $\begingroup$ Keep in mind you are also flying in a pressurized tin can miles high, and in minutes you reach air pressures appropriate to Colorado ski resorts, but without days to acclimate. These factors also attribute to the discomfort. $\endgroup$ – Harper - Reinstate Monica Oct 30 at 16:15
  • $\begingroup$ Airborne diseases also spread further in dry air. $\endgroup$ – Michael Oct 30 at 21:02
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What happens during a flight cycle is the humidity is high during departure, from ambient air and the moisture from the pax, but over time the air dries out because the bleed being supplied to keep the pressure hull "inflated" is coming from the engine compressors, which at 35000 ft has almost no humidity, and what humidity there is is being extracted by the Air Cycle Machines that condition the bleed (by heat exchanger condensate).

It's important to note the air cycle air conditioning process (787 excluded): Take hot high pressure bleed air, take away some of the heat with heat exchangers while keeping the pressure up, boost the pressure and temperature some more with an air cycle machine - more or less a turbocharger that feeds itself - , cool that off some more with more heat exchangers, then finally allow the significantly cooled but still very high pressure air to expand and drop down to just above cabin pressure. This process can take 500F air bleed from the engine compressor and chill it to -40 if you pass all of it through the air cycle machine (only some goes through, and gets mixed with bypassed bleed after, and the air cycle discharge itself is normally kept to not less than > freezing). Anyway, this process takes whatever humidity there is and removes it, leaving bone dry air. Moisture control is a big deal with Air Cycle Machines because condensate water that forms during the cooling process will rapidly erode the ACM turbine and ruin the ACM, so you have water separators that extract moisture that condenses from being chilled.

Then at altitude, there's almost no ambient moisture in the first place and you end up with really dry air from ambient being pumped in, how much depending on how leaky the pressure hull is (they can leak quite a lot) offset by humidity coming from all the pax. At high altitudes after a period of time the dry incoming air wins out and the pax humidity isn't enough and everybody starts to dessicate, again, depending on how leaky the pressure hull is. An older leaky airplane will get a lot drier inside than a newer relatively tight one.

As Mackk says, one of the big issues is that what moisture does get around the cabin ends up condensing on the inside of the pressure hull. It slowly gets absorbed into the insulation. Airliners that operate in humid environments down low will carry hundreds of pounds of water in the cabin insulation because of the condensation cycles from flights, and you will often see insulation set out to dry when airliners are in for heavy check inspections. What water doesn't soak into the insulation collects at the bottom of the fuselage and promotes corrosion (one solution is to pump dry conditioned air directly into the space between the insulation and skin - so called "zonal dryer" systems).

Anyway, you can see there is a this-or-that problem. Air moist enough to keep pax happy plays havoc with the interior. Air dry enough for a happy interior generates dry air complaints. In general, the industry tends to go with the latter.

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  • $\begingroup$ How did 787 manage it without the suite of problems for the operators? $\endgroup$ – JZYL Oct 30 at 16:30
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    $\begingroup$ I don't know much in the way of details of the 787, but it doesn't use engine bleed. It uses an electric based refrigeration/heating system, has a much more air tight composite pressure hull, so that moisture level can be managed more precisely at a higher level as per Dave's answer. $\endgroup$ – John K Oct 30 at 16:37
  • $\begingroup$ The ACM extracts the water from the air? Where is that water kept? Is it leaking out of the aircrfat? I guess that there can be a significant volume of water, especially when an aircraft operates in tropical environments where the humidity of the air is a lot higher. As the door closes after boarding this humid air is captured inside the cabin. $\endgroup$ – Brilsmurfffje Oct 31 at 13:34
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    $\begingroup$ It's either dumped overboard through a drain, or is commonly routed to an 'aspirator' that is used to spray the water on the heat exchangers to improve their efficiency. It doesn't have to be tropical. Look at an airliner on the ramp with packs on running off APU bleed on any humid summer day in NA, and you will sometimes spot a little drain hole with water spitting out of it. There is often still condensate moisture even after leaving the packs, and so you will typically find a last gasp water separator in the air conditioning ducting going into the pressure hull. $\endgroup$ – John K Oct 31 at 16:28
  • $\begingroup$ I'm sure the packs create condensate on the ground, but how found it create any at altitude. The dewpoint at 35,000 feet is like -50° C. Where would condensate come from? $\endgroup$ – TomMcW Nov 1 at 17:17
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Why is the air so dry

Its in part by design and in part a constraint of the system. All aircraft are at risk of corrosion not just big airliners. So keeping them dry is a good way to cut down on that risk. Even though they use fancy anti corrosive paints these days internal corrosion presents a serious risk as some areas may only be exposed during a maintenance check.

Big airliners simply pressurize the exterior air for the cabin so if the air outside is dry, which at 35,000 feet it is, then the air in the cabin is going to be dry as well.

and what do or can modern designs do to improve the air humidty on board an airliner?

Not much, the introduction of carbon composites to airframe design and construction has made corrosion less of an issue. In turn aircraft can tolerate a higher moisture content. This is effectively how the 787 was able to raise its moisture content to 14%. However the larger issue is where that actual moisture comes from, the air at high altitudes is not getting any more humid so the only way to increase the humidity is to either recycle the moisture you have or cary water and inject it. Both of these options present the same issue a heavy complex system and in aviation its all about weight. At 8.34LBS a gallon no one really wants to cart water around just to make the cabin a bit more comfortable, especially when that load can be used for revenue generating cargo or passengers.

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200 humans exhaling persistently in an enclosed, relatively warm, high pressure atmosphere creates large quantities of condensation on the cold interior surfaces & structure of the aircraft hull, creating a serious corrosion liability. Like computer mainframe facilities being carefully air conditioned & temperature controlled, de-humidifying the air is done deliberately for the benefit of the equipment, not the human occupants. Modern aircraft such as the 787 can have (slightly) increased ambient cabin humidity because of improved condensation mitigation & corrosion resistance.

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The humidity inside the plane has three constraints:

  1. at what point does the humidity become a corrosion risk for the plane? The 787 and presumably other future planes are now mostly "plastic" (composites) and can't corrode, so this limit is just in the process of increasing rapidly.

  2. what the hardware can in theory deliver: air at altitude is cold, and warming it lowers the humidity a huge amount. (This is why static electricity is a problem in the winter in cold climates.)

  3. the increased cost of carrying around literally a ton of water is considered by the airline along with whether the increased comfort it provides will get customers to pay more or be more loyal. In short, even if the plane can do it, the airline may decide not to.

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