Re: Passenger cabin pressurization

Date:         17 Sep 97 02:49:15 
From:         michael piersdorff <piersdorff.michael@ic.gc.ca>
Organization: Industry Canada
References:   1 2 3
Followups:    1
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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.

2.  Fuselage structures are designed to hold internal pressure, not
vacuum.  Indeed, the cabin pressure acts to stiffen the fuselage and
reduce compressive stresses which can be more critical than tensile
stresses because of the buckling problem.  Reduce the pressure by much,
and it would all collapse like a large plastic soft drink bottle does if
you suck the air out of it.

3.  In all the 15 or so years I spent as an aircraft structures and
certification engineer, I never heard of a cabin pressure reducing
pump.  That doesn't mean they don't exist, but I'd be very surprised if
they did, because of point 1.