Swedish Company Puts Airliners On An Aggressive Water-Weight Diet


Water weight from condensation can be a big problem for airliners, especially as flights get longer and load factors get higher. According to Swedish-based CTT Systems, each passenger exhales around 100 grams of water per hour, so 300 souls on board breathe out 30 liters per hour for up to 17 hours at 2.2 pounds per liter. You can say that the aircraft took off with all that weight, so what’s the big deal?

Well, that moisture rises with warm air to the crown of the fuselage, above the cabin ceiling. At 35,000 feet, when it contacts the minus-54-degree C aircraft skin, a lot of frost forms. That frost melts on descent into warmer air and gets sopped up by the insulation blankets. So, all that water weight that the passengers happily shed on a long flight pads the aircraft’s love handles—not good for fuel consumption and carbon emissions. Not to mention issues of mold, corrosion and possible water damage to wiring, much of which runs through the space above the cabin.

Swedish-based CTT Systems’ Anti-Condensation System sucks up the moist air from the crown area or cargo area and divides it into two airstreams. The first feeds past a slow-moving silica-gel rotary drum to absorb humidity. A specially designed “piccolo” duct, (with regularly spaced holes; like the instrument) releases the now-dry air between the ceiling panels and the external skin of the aircraft.

Electric heaters warm the second airstream before it, too, passes by the silica-impregnated rotor. This heated air sucks up the absorbed moisture from the gel and jettisons it overboard via the aircraft’s air-recirculation system or through the outflow valve.

In tests, three CTT-equipped easyJet Airbus A320s each “lost” more than 440 pounds in three months. As with most diets, when the Anti-Condensation Systems – which have an installed weight of 64 pounds – were deactivated over the next three months, those pesky pounds came right back. As part of the tests, there was also a 40 percent reduction in unscheduled antenna, sensor and computer replacements per 1,000 flight hours.

According to CTT, “For an Airbus A320 or Boeing 737, a [440-pound] weight reduction translates into … a 0.4 to 0.6 percent reduction in fuel consumption. It also reduces CO₂ emissions by more than 65 [metric tons].”

One less-mainstream but interesting application of the technology is on a Boeing 747SP operated under an 80/20 partnership between NASA and Germany’s Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe outer space. Operating at altitudes above atmospheric water vapor (which absorbs infrared radiation), the 747 carries a telescope mounted under a large upward-opening door in the fuselage. On long missions (up to 12 hours), the telescope is exposed to extremely low temperatures. The delicate surface of the primary mirror might be damaged by condensing moisture during descent.

For this application, CTT Systems developed an oversized version of the Anti-Condensation System, which is activated several hours before the end of the mission to warm the air and achieve a safe dew point. It stays on several hours after landing so the telescope remains dry as it assumes ambient surface temperatures.

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    • You make a good point Roger, the installed weight wasn’t mentioned. Assuming the weight added by the system and the weight reduction from moisture removal is a net zero, the reduction in maintenance items still might make it a good investment, but that remains to be seen. The thing I found most interesting was the functionality on the NASA bird.

    • The desiccant systems that I worked with in a past industry were less than 50 lbs. Double the size and and 50 more lbs for ducting, and I would estimate a desiccant system for this sort of application would be less than 200 lbs without paying any attention to weight savings. Engineered for a light weight design and realistically it probably adds less than 100 lbs.

  1. >> that moisture rises with warm air to the crown of the fuselage, above the cabin ceiling. At 35,000 feet, when it contacts the minus-54-degree C aircraft skin, a lot of frost forms. That frost melts on descent into warmer air and gets sopped up by the insulation blankets.

    Wait, where were those insulation blankets at the beginning of this paragraph? How did the moist airflow get to the fuselage skin in the first place? Also, anybody notice that the airflow moves from top to bottom during flight? Outlets in the ceiling. Air return registers at (or below) floor level. The air in an airliner cabin isn’t static. Doesn’t allow for much “warm air rises” activity.

    • The insulation from the blankets is only so effective. Plenty of air gets past it, carrying the exhaled moisture with it. The more passengers and the longer the flight, the more moisture that collects as frost on the cold aluminum skin. When it melts, the blankets do a great job soaking up the moisture on the way back down.

  2. So in the airline world one can install whatever new device one can conceive just like that ? I would love to put some Tesla Model X seats in the front of my 206, but it wouldn’t get a sign off at the next annual. How does this work ?

  3. Unless I am mistaken, one of the problems with cabin air on larger airliners is that it becomes drier as the flight progresses. This is caused by bringing in extremely dry outside air to pressurize the cabin, which results in passenger discomfort in the form of dry eyes, nasal dryness and throat irritation. People begin experiencing discomfort when relative humidity drops below around 30%. The effect is similar to the feeling people experience in the winter in northern climes, which prompts them to install humidifiers on their heating systems. I was under the impression that both Boeing and Airbus have humidity control systems on their planes to maintain the humidity at a comfortable level. This proposed system seems to work in opposition to the other.

    • Ironically, one of the benefits of this system would be to enable more humidity in the cabin for greater passenger comfort – if desired. The principle is that by keeping humidity down in the space between the cabin walls and the outer skin, you mitigate concern over corrosion and having to protect wiring harnesses. That allows margin for adding humidity in the cabin, though that would cut into the weight (aka $$) savings, which is perhaps why the airlines don’t seem to be so interested.
      As for issues of corrosion, that’s another design advantage for composite airframes.

      • Good point, Mark, thanks. While it does seem counterintuitive, it makes sense. As for the corrosion issue, even a composite airframe would still have the mold and mildew issue with the wet insulation. I do have one other question. The company claims a reduction of 65 tonnes of CO2 due to the weight reduction, but I’m curious about how many flight hours that would take. 440 pounds does not seem like much, but years ago when jet fuel got expensive, one of the US airlines removed the seat-back magazines from their cabins to save weight, so I guess it would add up.

        • Hi John,

          A rule of thumb is 2% to 3.5% of the extra weight in extra fuel burn per hour. Longer for long haul flights. So if your aircraft does 3000 hours per year, 440 lbs x 2.5% x 3000 = 33,000 lbs of fuel. Fuel burned to CO2 is 1:3.17, so 33,000 lbs of fuel would create 104,600 lbs of CO2, for 52 tons.

          So their claimed 65 tonnes is reasonable for one aircraft, but depends on number of flight hours per year and length of each leg.

  4. This reminds me tangentially of the Zeppelins USS Akron and Macon. Though theirs was the opposite problem of how not to lose weight. As airships burned fuel they got lighter and lighter and had to valve off expensive helium to maintain the same buoyancy. So the Akron and Macon had water condensors on the engine exhaust (the black rectangle running up the sides). Since burning a gallon of gasoline produces about a gallon of water, capturing this exhaust vapor allowed them to better maintain equilibrium.

  5. As an A&P, this points out the importance of ‘drip loops’ in wiring … something I’ll now pay more attention to.

    Anyone who has slept in a tent overnight has seen the moisture on the INSIDE of the tent just waiting to fall on you or get you wet, too.