If aeroplanes are pressurised, why should our ears still pop?

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Colin

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Colin  asked the Naked Scientists:

Hi Chris

I was listening to a recent ATNS show (22nd Aug) and you answered a  
listener's question about ears popping in an aeroplane. What you didn't explain however, is why the pressure changes inside  the plane.

I know that air pressure decreases as you get higher, but the plane  
is a sealed tube. Indeed, the infamous oxygen masks are installed because the air is so   thin up there that we can't breath.

I remember a recent Ryan-air flight having to land because the cabin  
pressure failed.

So why do the airlines drop the pressure and cause so many people so  much discomfort when they are still pressurising the plane anyway?

Thanks
Colin Donnelly

What do you think?

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Offline Bored chemist

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #1 on: 07/09/2008 09:54:49 »
They pressurise the plane, but not much. If you fill it with lots of air it weighs more so it takes more fuel to fly. You also need to pump more air in if you keep the pressure higher and that takes energy too. The airlines must think that people are prepared to put up with the discomfort in order to keep costs down.
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Offline LeeE

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #2 on: 07/09/2008 12:17:36 »
IIRC, airliners are pressurised to the equivilant of about 8000ft - most people are ok up to around that altitude.  Maintaining sea level pressure at cruise altitudes (up to around 36000ft) would result in greater stresses on the pressure hull, requiring it to be stronger, and this in turn would mean additional structural weight and therefore increased costs.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline daveshorts

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #3 on: 11/09/2008 12:27:02 »
Whilst the extra weight of the air would make a difference, but the arguments I have heard are more to do with the structural weight, and of course the energy required to compress the circulating air from outside.

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Offline graham.d

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #4 on: 11/09/2008 13:45:31 »
I think the weight saving by pressurising to 8000ft is about 260kg (about 580lb) for a Boeing 747-400. Although this does save fuel I think I would concur that this is not the major reason for not maintaining ground level atmospheric pressure. I doubt it is structural either because aircraft must be designed to not be damaged by depressurisation which take the pressure much lower than this. In any cases the difference in the forces between pressurising to 0.74 atmospheres and 1 atmosphere are not huge (outside would be about 0.1 atmospheres I think). I would go along with the size of the pumping gear and energy needed being the main reason. The cabin air does not just recirculate but has to have air drawn in from outside and suitably compressed. The stale air is vented. It probably takes a fair bit of energy and I suppose is a substantial sized bit of kit.

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Offline LeeE

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #5 on: 12/09/2008 15:11:05 »
Quote
I doubt it is structural either because aircraft must be designed to not be damaged by depressurisation which take the pressure much lower than this

Depressurisation doesn't increase stresses upon the hull - it relieves it i.e. equal pressure on the inside and on the outside = no pressure stress.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline graham.d

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #6 on: 12/09/2008 15:25:58 »
Yes, I realised that after I wrote this :-) D'oh!

The other bit is right though. The differences in the structural forces between 0.74 atmospheres and 1 atmosphere is not very high. However, I don't know for sure that there is no reasoning along these lines too, and that maybe it is a matter of building in more design margin. I could not find anything on the web though. If you do find anything definitive on the subject I would be interested to know.

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Offline LeeE

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...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline graham.d

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #8 on: 12/09/2008 22:07:18 »
Thanks Lee. The relevent bit is ...

"cabin pressures are typically maintained well below sea level pressures (equivalent to altitudes well above sea level) in order to minimise fuel costs and the costs of fuselage fatigue inspections, which are driven by the number and depth of pressurisation cycles."

So fuel costs are significant and the problem is fuselage fatigue because of inflating and deflating the "balloon" that is the fuselage. It looks like it is slightly less problematic if the "balloon" is not inflated quite so much. I now remember seeing a TV programme on air disasters which was related to this fatigue issue. The panels that made up the fuselage are riveted to a frame but each time the fuselage is pressurised the panels move slightly and fatigue the rivets. It was an aircraft in the Carribean and a huge section of the fuselage behind the cockpit blew away. I think, amazingly, only one person died and the plane managed to land safely.

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Offline wolfekeeper

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If aeroplanes are pressurised, why should our ears still pop?
« Reply #9 on: 17/09/2008 02:14:18 »
Normal air pressure is actually huge, it's 10 tonnes per square metre!!!

Concorde had big problems in this area- Concorde flew much higher (at 50 thousand feet compared to 35,000 feet for normal aircraft), and the air pressure was much lower there (about half). So the designers were forced to make the skin much thicker and this actually contributed to the high cost of flying Concorde. Concorde was actually about 3 times heavier than a 747, per passenger, and that's one of the reasons why.