In our Ask a Pilot series, pilot Spencer Marker answers one of your aviation-related questions each week. See past installments here and submit your own to Whitney@johnnyjet.com.
For those of you that have noticed, it’s been quite a while since I’ve posted a new Ask a Pilot. Well, to explain the delay: I got married to the woman of my dreams and went away on a fantastic honeymoon (stay tuned). But after all that excitement, I’ve gotten back to the daily grind of plying the skies and answering your questions. So that all being said, let’s dive in!
Can you explain the condensation in the plane in my video?
— Johnny Jet
Hey Johnny, thank you for asking such a great question! And providing a nice visual aid to help people understand exactly the phenomenon you’re talking about! The condensation you’re seeing comes from the usage of the airplane’s air conditioning system during hot and humid days. I’d even venture to guess your video was likely shot in Florida? During the normal use of the air conditioner on humid days, passengers can sometimes see condensation coming out of the vents. Let’s take a little deeper look at how airplanes air conditioners work, and why they create such lovely special effects!
Air cycle machines
In large jet airplanes, the cabin is conditioned with air that has been run through an air cycle machine (ACM). This equipment, located either in the belly of the aircraft or in the tail, is commonly referred to by pilots as the air conditioning packs, or simply, the packs. The packs are the refrigeration unit of the aircraft’s environmental control system (ECS). The ECS is responsible for the overall pressurization and temperature control of an airplane’s cabin.
As opposed to the air conditioning in your car, which uses a chemical refrigerant to cool the air, an ACM uses compressed air and physics to lower the air’s temperature before it’s ducted into the cabin. To accomplish this, unburned, compressed air is bled off the engine or APU (read more about the APU here). This highly compressed air is hot, sometimes over 150°C. So, the first stop to cool it down is a heat exchanger, which operates like your car’s radiator, but instead uses ambient air to lower the temperature. From here, it’s ducted to a compressor, which again raises both the temperature and pressure.
After passing through yet another air-to-air heat exchanger, this hot air is routed through an expansion turbine. A turbine is a wheel with vanes that used to extract work from compressed air. In an ACM this highly compressed, hot air, directed through this turbine, which spins like a pinwheel. In so doing, the air is allowed to expand, which drops the pressure and as a result, the temperature. Even during hot weather, the ACM can drop the air temperature below 0 °C. In addition, this spinning turbine is connected by a shaft to the compressor at the beginning of the cycle.
(You can actually hear the whine of the ACM from the jetway as you board Airbus and Boeing aircraft. You may think it’s an engine, but its actually the pack compressor spinning away!)
After leaving the pack, this cold air is mixed with untreated hot air also bled from the engine, mixed to the desired cabin temperature, and ducted into the cabin. Controlling the release of this air out of the airplane is how cabins are pressurized too, but that’s a story for another article!
So right about now, science-minded folks are thinking, “If the temperature is dropped that much in humid air, won’t the cooled air come out as a fog?” That’s correct: Left to its own devices, an ACM will drop the temperature below the dew point, causing the moisture in the air to condense and become visible.
Other than producing a nuisance fog, moisture and airplanes are largely not compatible. Left in a moist environment for long periods of time, an aircraft structure can corrode, leading to all sorts of issues. This is the principal reason cabin air is so dry. To combat this, engineers employ water separators in the ECS. This removes most of the humidity from the conditioned air, dumping it overboard (sometimes it’s reused in the heat exchangers) to ensure the air entering the cabin is dry.
Luckily, air at altitude is relatively dry anyway, so the water separators have no trouble removing moisture from the air before it’s ducted into the cabin. During humid days on the ground, however, the water separators can’t remove all the moisture from the air, so sometimes this cooled fog makes its way into the cabin, resulting in a harmless special effect. This foggy air occurs so infrequently that it doesn’t present a corrosion issue for the airplane.
To sum up
Thanks for asking such an awesome question, Johnny. I always love to dive into the nuts and bolts of how things work on the airplane and share them with my readers. The video you shared is a visible example of how the process of cooling the airplane works. While it might be concerning to some infrequent travelers, it’s simply water vapor, benign to both passengers and the airplane alike.
Thanks again for the question! Have you seen the fog on an airplane like Johnny has? If so, share your comments below. And remember, if anyone has a burning aviation question or something you would like cleared up, drop us a line at Whitney@johnnyjet.com to get your question featured in an upcoming Ask a Pilot column.