Why do they only part-pressurise planes?

14 September 2008

Question

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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