Re: Passenger cabin pressurization

Date:         17 Sep 97 02:49:16 
From:         Chris Dahler <dahler@iglobal.net>
Organization: gte.net
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Robert J. Montgomery wrote:
>
> Gerard Foley wrote:
> > jf mezei ("[non-spam]jfmezei"@videotron.ca) wrote:
> >
> > : 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 ?
>
> *snip*
>
> >   There was usually a little bump upward in cabin pressure when
> > the doors were closed.
>
> Kind of a related question. I almost remember hearing, from some
> flight crew friend of mine, that most airlines actually decrease
> pressure prior to take-off. This way, they have to accelerate less
> mass during the takeoff process. Is this ever true? I've felt pressure
> changes in planes on the tarmac prior to takeoff (ear pop), but
> this may be caused by Gerry's pressure bump.

The figure for virtually every modern airliner is 8000 feet cabin
altitude at max certificated altitude.  This results in varying
differential pressures depending on what altitude the aircraft is
certified to; a 727, certified to a ceiling of 42,000 feet has a maximum
operating differential of 8.65 psi, and there are relief valves in the
fuselage which will begin to open at around 9.5 psi.  Other aircraft,
such as the 737, are certified to lower ceilings and so have a lower
maximum operating differential; 7.45-7.55 psi is all that is needed to
maintain 8000 feet at a service ceiling of 35,000 to 37,000 feet.

Most cabins are pressurized on the ground to around 0.1 psi differential
(in other words, the cabin altitude is lower than the airport elevation
by about 200 feet) in order to allow for smooth changes in cabin
pressurization after takeoff - this is done entirely for passenger
comfort.  The limit is generally 0.125 psi so that the doors may still
be opened.  While this doesn't sound like much, I've tried just opening
the cockpit window in a 727 with the cabin pressurized to 0.125 psi, and
it was quite a strain.

Modern airline pressurization is maintained by pressurization
controllers on which the crew sets two values prior to takeoff: the
planned cruising altitude and the destination airport elevation.  If the
cruise altitude is low enough that the maximum allowed differential will
maintain a cabin altitude at the destination field elevation over the
course of the flight, then the controller will pressurize the cabin
after takeoff to the destination field elevation and leave it there; you
won't feel any pressure changes in your ears during descent.  This
typically only happens when the flight is quite short and the cruise
altitude is 25000 or less.  If the controller can maintain a cabin
altitude which is lower than the destination field elevation, then it
will just pressurize the aircraft to the maximum differential and then
actually climb the cabin during the descent to the destination.  This
will typically happen when the destination is some high elevation
airport, such as Denver.  On most flights, the maximum differential will
only be able to maintain a cabin altitude which is higher than both
departure and destination airports.  In these cases, the controller
pressurizes the aircraft to the maximum differential during the climb
and then descends the cabin during the descent.  The controller will
generally hold the rate of climb to 500 fpm and the rate of descent to
300 fpm.

Chris Dahler
dahler@iglobal.net