The air at altitude is very dry, I believe that ultra long haul airplanes like the 777’s have humidifiers on board supplied by the on board water source. Newer airplanes recycle more cabin air to conserve fuel, airplanes like the DC-9’s or 727’s did not recycle any air, all air coming into the cabin was fresh air from outside.

It is true about the reason to keep the humidity level low to prevent corrosion, condensation can form with higher levels of humidity and the condensation drips down to the belly of the airplane

Because the outside air at 39,000 feet is about –40 degrees F, all airliners have insulation just inside the outside skins, this insulation over time can absorb and hold this condensation creating the source for corrosion and also add weight to the airplane. I have seen some airplanes like the 727’s that had sections of the belly skins replaced because of excess corrosion.

Airplanes are pressurized by taking high pressure air off of the engines, known as bleed air and ducting it into the cabin. The less air they have to bleed off of the engines, the more air is available for thrust, so this is why the recycle more air today than they did in older airplanes, to save fuel. This pressurization air is regulated by valves that let the air out of the cabin called outflow valves.

Dehydration on long flights can be a problem, I try to drink about 12 ounces of liquid every 2 hours or so to keep hydrated.

For those interested, all toilet waste goes into holding tanks where it is pumped out into a special truck when on the ground. All water from the lav sinks and galley drains is dumped overboard via a heated outside drain mast under the fuselage of the airplane, these fluids are atomized into a fine mist and evaporate before reaching the ground.

Blue ice is caused by a leaking lav toilet tank drain seal, so the toilet fluid just leaks out, freezes and stays on the fuselage until the airplane is at a lower and warmer altitude and falls off of the airplane.

Mr. Elliott