Re: Passenger cabin pressurization

Date:         06 Oct 97 02:14:23 
From:         Ken Rose <rose@acm.org>
Organization: Netcom On-Line Services
References:   1 2 3 4
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michael piersdorff wrote:
>
> Robert J. Montgomery wrote:
> > Gerard Foley wrote:
> > >   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.
>
> 1.  The total mass of air inside a 747 cannot amount to more than a few
> kilograms - probably less than that of the airpump it would take to
> remove it.

On the contrary, the mass of air inside a 747 is substantial.  If the
fuselage is a cylinder 20 feet in diameter and 200 feet long (very
rough,
but in the ballpark), then it has a volume of (20/2)**2 * 3.14 * 200 =
62800 cubic feet.  A cubic foot of air weighs roughly 1 ounce, so
62800cf
of air weighs 3925 pounds, or about 1785kg.  I think you probably could
pump a lot of it out with a pump that weighs a lot less than that, but
your other reasons for not doing it still apply.