Optimizing humidity in aircraft
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Fuselage condensation occurs in all aircraft. Passive means are the standard fitted on aircraft to prevent moisture issues. But in many situations those no longer are sufficient. In the past 20 years, airlines configure more seats (seat-densification) and operate with higher load-factors, causing water accumulation in the fuselage / structures. In addition, flights are scheduled with shorter turn-around and increased operational lenght. This result in more total flight hours per year (i.e. less drainage hours on ground).
During flight each passenger exhales around 100 grams of water per hour. Humidity is approx. 15 % RH in a high-density cabin. The temperature on the inside of the surface is below the frost point. Water vapor in the air condenses to frost when it diffuses and gets into contact with the fuselage surface. The frost melts to water during descend. Passive means are designed and installed to lead water to the belly and then to drain condense water on ground. Most of the water disappear but not all water and therefore it accumulates in the fuselage structures.
All aircraft carry trapped condensate water in blankets and fuselage structures. Condensation can increase the aircraft weight by quarter of a ton, depending on the number of passengers, type of operation and climate zone. How much it actually is in a specific aircraft depends on type of operations and geographical hub conditions. A high-density carrier in the northern hemisphere typically carries more water than an airline based in the Gulf region.