27

A lot of consideration is placed on the airframe (which has to hold for 20+ years) rather than the passengers. One of the primary reasons is to slow corrosion of the aluminium airframe. This is also why the Boeing 787 is a lot more humid - corrosion is no longer an issue in a composite fuselage (or a lot less at least). Boeing and the Economist sum it ...


16

There are a couple of things that contribute to the dryness of cabin air: Cabin air is drawn from engine bleed taps in the high pressure compressor. This is ambient air, which at the flight levels is normally quite dry and then heated and compressed, which will lower the relative humidity, making it seem even drier. The air cycle machine subsequently ...


7

When I flew the C-141 and C-17, their environmental systems weren't intentionally dehumidifiers (designed to reduce the overall humidity), but they did push the air through a fabric bag called the 'sock' to capture droplets that formed during cooling. As air continued to flow, that moisture could evaporate and rejoin the stream, or be vented overboard ...


5

A Reuters article confirms the humidity just comes from the passengers, on the 787 at anyway: (...) the air is dry and moisture comes mostly from passengers. And this Flight Global article: (...) the system does not add moisture through active humidifiers. This APEX article has no discussion of higher cabin pressure being an influence on humidity. ...


5

These sources say that most airliners maintain a humidity of 4-7%, while the 787 maintains 10-15%. Of course these values can vary with conditions, and other sources put them lower or higher. The cabin and cockpit will not have very different air qualities. While some cockpits may humidify the cockpit air slightly, the cockpit tends to have a greater amount ...


2

To add to the answers above: the other main reason (besides condensation) for keeping humidity low is to prevent the growth of mold and bacteria in the cabin and the air conditioning system, which over time may pose a health hazard to the plane's occupants. And I can't stress enough that the air is not de-humidified, but that the outside air (which is ...


2

There may be two factors at work. First, there may be less air exchange, which would leave more moisture in the aircraft from people. Reducing air exchange reduces the power required to pressurize the aircraft. As I recall the 787 is a no-bleed system, with electrical compressors utilized, resulting in a claimed 3% fuel efficiency. Accordingly, I would ...


1

Coming with some ECS/Pressurization background in engineering tech support, the only factor I can think of where higher pressure differential would influence humidity, which is all coming from passengers, would be related to leak rate of the pressure hull (they all leak, mostly through door seals etc). I might expect higher differential to result in a ...


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