Re: Passenger cabin pressurization

Date:         10 Sep 97 19:38:45 
From:         rdd@netcom.nospam.com
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In article <airliners.1997.2115@ohare.Chicago.COM> "[non-spam]jfmezei"@videotron.ca writes:
>I have seen mentioned often that modern airliners maintain a cabin
>pressure of 8000feet when flying at cruise altitude of 30-40k feet.
>
>Is this an urban myth or actual precise figure that applies to all
>airliners ?
>
>Also, since most flights (even 1 hour hops)  on jets tend to go to 30k
>feet and above, is it fair to assume that on just about all jet flights,
>the cabin pressure will go down to the 8000ft limit ?
>
>Or is the cabin pressure really determined by a pressure differential
>maximum which would mean, for axample, that Airplane X, able to maintain
>an 8000ft cabin pressure while flying at 40k feet, would maintain
>a higher cabin pressure (lower alttude) if it were to fly at only at 30k
>feet ?

Gee, JF, you've been posting to this group long enough to have seen the
very detailed discussions over the last two years.

But lest your memory has suffered, a typical pressurization system maintains
a pressure differential of 8.5 psi, or whatever is closest to the landing
altitude.

So if you'll be landing at sea level and cruising at 22,000', your cabin
altitude will be 0. 6.2 psi + 8.5 = 14.7.

If you will be landing at 6000' and cruising at 22,000', your cabin altitude
will be 6000'.   6.2 psi + 8.5 = 14.7, 14.7 > 11.8, so there's room for
control.

If you will be landing at sea level and cruising at 40,000', your cabin
altitude will be 2.7 + 8.5 =11.2 = 6000'.

These are all characteristics which are typical of jet transports.  The
only value which may vary is the max. pressure differential.  8.5 is
typical of most jet transports; Concorde's is higher, something like
11 psi.

>Reason for question: for scuba diving, the actual cabin pressure
>experienced by passengers can make a big difference for Decompression
>Sickness following dives.

...and since you will have NO control over it, plan for the worst case
scenario, which would be depressurization followed by descent to a lower
altitude, and potentially sustained flight at 12000' or lower.

--
Robert Dorsett                         Moderator, sci.aeronautics.simulation
rdd@netcom.com                         aero-simulation@cactus.org

           "Bother," said Pooh when his engine quit on take-off.