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ear-pop edification
June 23, 2009 11:35 PM   Subscribe

Airplane ear-popping. Why?

The cabin is pressurized, right? So why do your ears pop when you're taking off and landing?
posted by lockestockbarrel to Travel & Transportation (13 answers total)
 
Because the cabin is pressurised to a pressure significantly below 1 atmosphere.
posted by pompomtom at 11:46 PM on June 23, 2009


Because they don't pressurize it all the way to ground level. A bigger pressure difference requires more hull strength and weight, and also sometimes your destination is higher up than your originating location (Miami to Denver, for example).
posted by that girl at 11:46 PM on June 23, 2009


Some quick Googling points to this: The cabin is pressurized to only 75% of the pressure at sea level, not the same pressure as it is at sea level.
posted by Geppp at 11:46 PM on June 23, 2009


Doh, that girl beat me to it
posted by Geppp at 11:47 PM on June 23, 2009


also in link form.
posted by pompomtom at 11:50 PM on June 23, 2009


In case the cabin pressure angle doesn't cover it, what's reacting is your Eustachian tubes, same as in any pressure change, including diving underwater.
posted by hypersloth at 1:02 AM on June 24, 2009


Also even if the cabin was pressurised to 100%, the aircraft is designed to be flexible and will expand or contract to varying percentages during flight. Thus the pressure will change no matter what pressure the cabin is set to at ground level.
posted by fearnothing at 1:36 AM on June 24, 2009


I think everyone has it covered pretty well... if you want to read about how it works mechanically and what things are set at during different phases of the flight, there's this.

The funny thing is if you take -50C air from outside at cruise altitude and pressurize it to 1 atmosphere, it will heat adiabatically to about 40 to 50C (104 to 120F) once in the cabin. That would require some serious air conditioning, and of course you've also got to pay for an old guy to stand there and make sure no one messes with the thermostat.
posted by crapmatic at 1:52 AM on June 24, 2009 [2 favorites]


In addition though, although the cabin is pressurized (either while taxiing or right at take-off), that pressure won't remain absolutely constant throughout the flight -- which is why as you descend your ears pop over and over. They can start popping descending from 30,000ft to 25,000 ft, even though the plane obviously is still pressurized to keep everyone breathing at 25k.
posted by davidnc at 7:39 AM on June 24, 2009


Also even if the cabin was pressurised to 100%, the aircraft is designed to be flexible and will expand or contract to varying percentages during flight. Thus the pressure will change no matter what pressure the cabin is set to at ground level.

Actually, although the cabin is pressurized, it is not sealed. Pressurized air is bled from the engines and constantly fed into the cabin, and an outflow valve (often in the tail) is set to maintain the desired cabin pressure by regulating the amount of air flowing out of the cabin.

The setpoint maintained by that valve is controlled by a fancy pressurization controller that slowly decreases the cabin pressure as the aircraft climbs, and slowly increases it as the plane descends. That's because, as others have mentioned, the plane is not structurally able to handle the difference between sea level pressure inside, and atmospheric pressure outside at 30-40,000 feet.

It's not even as simple as keeping the same pressure as outside below a certain altitude and a constant pressure above. The outside air pressure changes too fast at low altitudes for the comfort of the passengers' ears, and the controller does a good job of keeping the rate of cabin pressure change to a comfortable value. So the cabin is pressurized practically from takeoff onward.

The ones I have seen even have a little window where you set the landing altitude (feet above sea level) so that the cabin pressure can reach exactly the outside pressure at landing. Otherwise one might end up landing with the cabin still pressurized, which would make it impossible to open the doors, and you'd have to dump the excess pressure rapidly.
posted by FishBike at 8:01 AM on June 24, 2009 [1 favorite]


Actually, although the cabin is pressurized, it is not sealed. Pressurized air is bled from the engines and constantly fed into the cabin, and an outflow valve (often in the tail) is set to maintain the desired cabin pressure by regulating the amount of air flowing out of the cabin.


A tiny amount of air also leaks through worn seals in doors and other openings in the aircraft. When smoking was permitted on planes, the leaks could be easily found by the brown tobacco stains at the point of leakage. Outflow valves had to be regularly cleaned because of tobacco tar gumming them up.
posted by JackFlash at 8:23 AM on June 24, 2009


In case the cabin pressure angle doesn't cover it, what's reacting is your Eustachian tubes, same as in any pressure change, including diving underwater.
hypersloth, not to be snarky (for once ;) ), but the cabin pressure angle is what your eustachian tubes are reacting to.


Also even if the cabin was pressurised to 100%, the aircraft is designed to be flexible and will expand or contract to varying percentages during flight. Thus the pressure will change no matter what pressure the cabin is set to at ground level.

fearnothing, I think others have pointed this out, but the pressurization isn't "set once and leave alone"; it's continually controlled. Therefore, changes in the plane's volume will be compensated by the pressurization system.

Otherwise, the aforementioned leaks, or the radical event of a serious rupture that is quickly sealed, would leave the pressure continually dropping in the plane (regardless of elevation, due to the venturi effect). That would be a Bad Thing. On shorter flights, without emergencies (that is, in a Cessna cockpit, for instance), no big deal. On a transatlantic flight, it is a big deal, and the commercial airliners are prepared - that is, they actively control pressure.

But not 100%, instantly, as noted. The pressure pumps can't necessarily keep up with the rapid drops caused by steep ascent/descent, so your ears still pop, while the system plays "catch up".
posted by IAmBroom at 9:53 AM on June 25, 2009


I meant if the cabin pressure angle [angle as in answer] doesn't cover it [your question], what's reacting [to the cabin pressurization and change in atmospheric pressure] is your Eustachian tubes..
so good news, no need not to be snarky!
posted by hypersloth at 3:04 AM on June 27, 2009


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