So, when the cabin is sealed, the interior pressure is equal to the atmospheric pressure at sea level. When the plane lands, the exterior pressure is equal to the interior pressure. So why does the plane need to be pressurised for the duration of the trip?
In: Engineering
At 30,000 feet there is very little air. It’s low pressure and would be very hard to breathe. Without pressurizing the air we would have to breathe very quickly in order to get enough oxygen and it would be extremely uncomfortable. Even with the pressurized cabins I get horrible ear and sinus pain. Without the pressure I’d be in agony.
At 30-50,000 feet cruising altitude (higher than Mt. Everest) the atmosphere is perilously thin.
This is great for the plane since it dramatically decreases wind resistance, but it’s not so great for the people inside who like to breathe.
Such low atmospheric pressures cause hyperventilation and possible loss of consciousness with prolonged exposure, especially if you’re from a low-altitude region and not acclimated to it.
The plane isn’t perfectly airtight (nor would you want it to be with 200 people breathing inside) so the cabin pressure needs to be continually vented and boosted.
A lot of people have misunderstood that question. So i’ll leave out the “why humans do bad at low oxygen pressure and why we shouldn’t do that” bit.
The cabin is not sealed – the air is actually being constantly replenished, under pressure. Think of it like a balloon with a pinprick leak. If you blow into it faster than it is leaking out, then the balloon will stay inflated. This is important because as people breathe in the cabin, oxygen would slowly be replaced with carbon dioxide. You could filter this, spacecraft style, but that’s heavy and expensive. Much easier to just replace the air with fresh (compressed) outside air.
Even if that wasn’t an issue, having a perfectly sealed cabin would spell trouble on flights to different cities. They are at different elevations, and this different ambient pressures. A flight from Los Angeles to Denver would leave the airplane pressurized relative to the outside, creating hazards and a very uncomfortable sudden jump when the doors are opened.
>So, when the cabin is sealed, the interior pressure is equal to the atmospheric pressure at sea level.
That is a pressurized cabin. The alternative is on where air can flow freely in and out so it is not sealed.
If you had a perfect sealed cabin and just closed the door at takeoff and nothing leaked out during the flight you have describes a pressurized cabin with no air replacement. The pressure at altitude is lower then att takeoff so if the pressure is higher inside if is is sealed ie a pressure cabin.
You also need to refresh the air because the oxygen level would drop and carbon dioxide levels would increase. So you compress air from the outside and let it in it in the front and let air leak out in a controlled way.
In practice, you do not use sea level pressure but the pressure that is at 8,000 ft (2,400 m) or below. The airplane expands slightly when the pressure inside them is higher than the outside and is stresses the airframe. So you need thicker material, more maintenance and perhaps a shorter life for the aircraft it the pressure inside was higher.
> cabin is sealed
The cabin is not sealed. There is a small vent at the back. You can see it here, with the red border around it: https://i.imgur.com/KYkePn7.jpg
The engines provide fresh air to the cabin (or in the case of the 787, electric compressors). The air pressure inside the aircraft is equal to the pressure at about 8,000 ft. Keeping the pressure lower than ground level puts less stress on the airframe and is less weight in air to carry around.
Smarter everyday did a video on this very subject, and it very enlightening:
Another way to think about this is climbing Everest (or any 8000m peak). Above 8000m is the death zone. If you could fly a helicopter from sea level to Everest peak, you would pass out in a few minutes (and die soon after). Climbers acclimate themselves to the altitude over WEEKS by going up partially and down many times (base camp is already ~18,000 ft).
I have experienced this to a much lesser degree by traveling from my home which is only about 400 ft above sea level to Breckenridge Colorado at a house about 11,000 ft above seal level. Taking a set of stairs or two, or climbing up a hillside is excruciating the first couple of days because your lungs and blood are used to “thick” oxygen rich air and are now have much less oxygen.
OK, BOT, is that ENOUGH?????
Because if it wasn’t pressurized, you’ll be sick, you will get high attitude symptoms, like headache, vomiting, heavy breathing, like if you were on the top of a mountain.
It can cause death for people suffering from other diseases as they are weak and need medical attention.
At those altitudes the pressure is insufficient for your lungs to inflate properly. A lot of people will say ‘there is less oxygen…’ but it is more right to say the lack of pressure makes creating sufficient gradient in your lungs impossible. If you simply pressurized the outside air (with a mask or something) the pressurization process would get enough molecules together for you to get enough oxygen. Same thing when you go under water, the air is *too* pressurized so you have to breath a tank filled at regular pressure.
EDIT –
For the inevitable people who read this and go “wait, I was always told…”, you were told wrong. Think about it, if there isn’t enough oxygen to breath, how in the hell is there enough oxygen for a jet engine to combust? Why does a turbine work at that altitude but a ICE engine struggles? It is *pressure,* a jet engine is little more than an air compressing machine. If you have a tool that can compress the air at 30,000 feet by vacuuming up air molecules from far ahead of the plane the makeup of that gas is the exact same as it is at sea level, the only difference is you had to vacuum it up. If your lungs were a powerful vacuum you could easily breath at 30,000 feet. The problem is your biology cannot vacuum up enough air at 30,000 feet and as a result despite breathing hard you won’t get any gas into the lungs. Hence pressurization.
Because people like to be able to breathe. And they especially want their pilots to be able to breathe.
As air pressure decreases, less oxygen is available to our bodies. In aviation, there is the notion of Time of useful consciousness (TUC), which his how long someone acclimated to sea level will be functional enough to operate an aircraft. As low at 10,000 feet you start to see deficits, at 15,000 you have about a half-hour, and 35,000 you have maybe 30 seconds.
Because the air pressure is extremely low at typical cruising altitudes. You’d lose consciousness from lack of oxygen in less than a minute, and would die not long after. That’s why planes have oxygen masks that drop down in case the plane loses pressure. It’s also cold. Like, really cold. Upwards of -60C cold.
As an aside, plane cabins are pressurized, but not to sea level. Usually they’re pressurized to somewhere between 6,000ft – 8,000ft. This is enough to be comfortable for people while also reducing stress on the aircraft from the pressure differential.
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