Last week the world was treated to an unexpected spectacle - the sight of an airplane cabin with a big hole in it open to the sky. We can imagine that this spectacle was even more unexpected to people who were on the flight. If you saw photos or video of the hole, you may have also been struck by how little there is between "inside" and "outside" in an airplane. There is a piece of plastic headliner on the inside of the plane, some insulation and then a thin aluminum skin on the exterior of the plane. That's it.
It brings up an interesting question - what is going on inside an airplane cabin when it is cruising at 33,000 feet? It turns out that passengers are flying in something that vaguely resembles a space capsule. Let's take a look at how the space capsule works.
The first thing to understand is that people dressed in normal clothing definitely cannot survive at 33,000. This altitude is roughly the equivalent to standing at the summit of Mount Everest. If there were some way you could stick your arm out the window at 33,000 feet, the first thing you would notice is that it is incredibly cold - minus 40 degrees F or colder. The second problem is incredibly low air pressure. The pressure is so low that people would pass out very quickly from lack of oxygen. The air at that altitude and temperature is also extremely dry.
So how are we able to sit in an airplane's comfy chairs at 33,000 feet feeling like we are sitting in someone's living room?
The first thing that has to happen is pressurization. The air at sea level is about 14.7 PSI (pounds per square inch). The pressure at 33,000 feet (roughly 6 miles up) is approximately 4 PSI. Something has to be done to increase the pressure, or people would quickly pass out from lack of oxygen at 4 PSI. Fortunately, the jet engines on the aircraft act like big air compressors.
If you take apart a jet engine and look at it, it has four main sections. At the front, where the air is coming in, there is the compressor stage. Blades suck in air and compress it. The fuel is injected into the compressed air and ignited in the combustion stage. The air expands greatly from the heat of combustion, and flows through another set of blades, turning them as it passes through. And then the exhaust gases flow out of the engine to create thrust to keep the airplane in the air.
By creating an opening in the engine between the compression stage and the combustion stage, high pressure air can bleed out of the engine and feed into the cabin to pressurize it. Because this air has just been pressurized, it is hot. Therefore, the ventilation system on the plane will first cool it down (using the extremely cold outside air that is readily available) to a comfortable temperature. The air pressure inside the plane is not sea level pressure - it is more like Denver pressure. You can think of the airplane's cabin like a big pressurized tube. See this article for lots more details. See also:
Now we have a cabin that is pressurized and warm. But because the outside air is so incredibly dry, some consideration has to be given to humidity. Fortunately the plane is full of humidifiers. People give off moisture every time they exhale, and also through perspiration. So the dry air from outside is mixed with the air already in the cabin and recirculated. The ratio of new air and existing air is typically 50/50. The recirculated air passes through filters that remove any airborne particulates. The air in the cabin is still dry, but not nearly as dry as it could be.
What happens if cabin pressurization fails? This can occur if the airplane's skin ruptures or a window breaks. I have been on a flight where the co-pilot's window cracked, and that was enough to depressurize the cabin. When that happens, the masks overhead will deploy and the pilot will immediately start descending down to a safe altitude like 8,000 feet. The masks get their oxygen not from pressurized tanks of oxygen (they would be too heavy) but instead from a chemical reaction involving something like potassium chlorate. When heated, potassium chlorate gives off lots of oxygen and a chemical oxygen canister like this is very light, relatively speaking.
The next time you board an airplane, take a moment to marvel at what is happening. You will be sitting in a comfortable chair at 33,000 feet, just like you might sit in your living room. An amazing amount of technology makes that possible.