Originally Posted by
cordelli
I also believe that the air introduced into the plane is humidified a bit, so there would be some water vapor added to it, as that circulates and is exchanged some moisture may escape, causing the weight to drop a bit.
It is my understanding that the 787 has such a system, but that existing airliners do not. That is one of the reasons that the airliner industry gives for recycling a portion of the air, to keep the humidity from being lower than it already is. I read that the reason existing planes don't humidify is that more moisture would lead to more corrosion of the frame and skin, which is not so much of an issue with the composite 787.