Science Questions

Why do they only part-pressurise planes?

Sun, 14th Sep 2008

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Question

Colin asked:

You discussed pressurising air in the aeroplane. Listeners were saying they’d been on planes and their ears had popped. Why do they pressurise the plane to only 8000 feet? Why don’t they pressurise it to atmospheric pressure?

Answer

Dave -  They did start doing this on the first jet airliner which was the comet. They also had lovely square windows and the aeroplane kept getting pumped up to atmospheric pressure and kept rising high up in the atmosphere. It got a big pressure on the structure coming down again and going up again. That stress on the airframe slowly built up cracks which got longer and longer until the cracks ran between all the windows and it opened up like opening postage stamps.  They actually lost several planes due to this.  Since that they’ve made the windows much more curved and they’ve also started not pressurising the plane all the way up so you’re putting less stress on it. You could design a plane which would survive the high pressure but it would involve making them much heavier than you’d want to and they want to make it as cheap as possible.

Chris -  You’re carting loads more weight into the air which is costing you more fuel and in these days of high fuel prices that’s a bad thing.

Dave -  The other thing is that to compress that air - you need more air because you’re constantly taking more air and compressing it from a low pressure to a high pressure inside the plane - takes lots of energy and more fuel

Chris - Graham.d on our forum calculated that the actual weight of the molecules themselves would contribute an extra 250kg. Basically an average British large person or several people my weight travelling for free if you don’t pressurise to atmospheric sea level pressure all the way up there. That’s pretty interesting.

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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? Colin , Sat, 6th Sep 2008

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. Bored chemist, Sun, 7th Sep 2008

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. LeeE, Sun, 7th Sep 2008

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. daveshorts, Thu, 11th Sep 2008

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. graham.d, Thu, 11th Sep 2008



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. LeeE, Fri, 12th Sep 2008

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. graham.d, Fri, 12th Sep 2008

Have a look at:

http://en.wikipedia.org/wiki/Pressurized_cabin#Pressurised_flight LeeE, Fri, 12th Sep 2008

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. graham.d, Fri, 12th Sep 2008

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.
wolfekeeper, Wed, 17th Sep 2008

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