Why Are Airplane Cabins Pressurized Rather Than Just Sealed And Oxygenated?

Question

Everyone knows that passenger jet’s cabin must be air tight to maintain a breathable atmosphere for the passengers, BUT what I’d like to know is, why is the cabin pressure increased so greatly for flight? Especially when at higher altitudes, with thinner air, there is less athmospheric pressure pushing in on the aircraft hull from outside.

Wouldn’t it be possible to maintain a breathable atmosphere inside the plane with air pressure equal to that on the ground if the cabin were only airtight and supplied with oxygen?

Answer 1

Yes, that would be possible but it would have a lot of adverse effects.

By the way, the cabin is not totally airtight, it does deliberately leak so that freah air can replace the lost air, which ensure some circulation.

Normally, the pressure inside an aircraft is less than one atmosphere at altitude. An airplane cruising at 35000 ft will have a cabin pressure equivalent of outside air at 9000 ft or so (there is a trend to make that cabin pressure altitude lower, by the way, for added comfort. At the same time, some airlines do not want to pressurise the cabin to a lower altitude because this requires more power from the engine, and thus use more fuel.

To keep an aircraft airtight and at 1 atmosphere with pure oxygen added would:

- increase the stress on the airframe as the pressure differential between the cabin and the outside would be greater, and that could fatigue the metal
- require pure oxygen supply
- require CO2 scrubbers
(oxygen supply and CO2 scrubbers would increase weight, cost, and maintenance effort)
- increase risks as pure oxygen can make nearby flammable material nearly explosive

The airtight-with-oxygen-supply solution is reserved for spaceships, and those cost a lot more to operate than airplane as a consequence.

Answer 2

Storing and generating oxygen is somewhat dangerous, as pure oxygen is highly reactive. It's both safer and cheaper to provide a breathable atmosphere by compressing the existing air to a reasonably comfortable level.

When you ask "why is the cabin pressure increased so greatly for flight?", are you thinking that cabin pressure is greater than 1 atmosphere? It's not. Commercial aircraft cabins are normally pressurized to an equivalent altitude of around 8000 ft. I think that's about .75 atmospheres.

Answer 3

Kind of like a submarine going deep...the pressure eventually would crush the sub if the pressure wasn't stabilized. It's the opposite with an airplane. The higher you go the thinner the air and less pressure than when the plane was on the ground. If it wasn't pressurized as the plane went up all the air inside the plane would rush out and the occupants would have no air....or at least a lot less oxygen...also very cold....do you remember when Payne Stewart's, the golfer, plane lost pressure...the same type of thing.

Answer 4

Because, even if the plane were sealed, but not pressurized, the plane's hull would flex out just enough to drop the pressure in the cabin enough to pop everyone's ears, and thin out the air. So, they pressurize the plane before take-off, to always be sure the pressure will hold, and then climb to the higher altitude.

Answer 5

Mainly because Hollywood kicks back some money to the airplane manufacturers. Imagine a movie where a hijacker shoots out a window and all the passengers simply wimp out and die from asphyxiation instead of being sucked out through the little tiny window after herocially clinging to their seat while floating horizontally in the air for 2 minutes, and then having the hero plug the hole and save the rest of the passengers.

Geez, you trying to kill movies?

Answer 6

There would be a CO2 build up. Also ozone from the electronics. If the plane is new, formaldehyde from the carpet (that is part of the new car smell that everyone seems to like). And the ever popular body odor of up to 200 people trapped in a confined space.

Answer 7

My guess is it probably has to do with temperature...

There is a direct correlation to pressure and temperature of gases.

The warmer a gas, the more pressure. The cooler a gas, the less pressure.

Don't take my word for it though, I don't know anything about this stuff.

Answer 8

Sure. The problem is the discomfort that most passengers would experience without the pressurization.