View Full Version : Pressurisation question

10th Feb 2018, 00:09
A silly question from a dumb SLF. As my holiday flight left the 38,000 ft cruise this afternoon my ears started popping pretty much straight away as usual, implying an immediate increase in cabin pressure. Then it occurred to me that I thought the cabins were typically pressurised to 8000 ft equivalent or so, so why should there be any change in cabin pressure before the aircraft descends through 8000 ft beyond which I would expect it to increase with environmental pressure ? I know, I know, a really silly physics question, but I'm genuinely interested to understand what's going on. My hobby flying rarely gets me to 6000 ft so it's not usually an issue. Perhaps a professional driver out there can explain it for me ?

10th Feb 2018, 01:54
Thanks Tango Man. Passenger comfort then. Also the reason why we don't accelerate and decelerate tube trains more than about 1.2 m/s/s, otherwise they end up in a big pile behind the driver's cab door !

10th Feb 2018, 09:24
So basically in the last 8000' to surface of aircraft actual altitude the airplane will be actually doing anything from 1500 feet per minute to 800 feet per minute. These are both above what is comfortable to a non aclimitised passenger to non pressurized flight.

I remember many years ago when I used to sell unpressurised commuter aircraft in Canada, we used to counter that argument, somewhat disingenuously, by saying that the lift at the CN tower went up and down at 1200 fpm. :O

10th Feb 2018, 09:59
Thank you Basil, very neat. As for DaveReidUK and your heinous crime of mis-selling? You are due some terrible punishment when you finally check in at the last terminal of life ... :=

11th Feb 2018, 01:59
The pressurisation system, keeps the internal pressure some psi above outside pressure ( for example 5 Psi above outside) at 38000ft, the pressure outside would be nearing 0psi (maybe 1 or 2), so internal pressure would be around 6 or 7 psi . As the aircraft descends,the outside preure will increase, so internal will increase also, until it reaches about 14 psi (average ground level pressure)

This is a very simplified explantion... but close enough