Re: Cabin pressure

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         01 Jun 95 05:00:58 
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In article <airliners.1995.686@ohare.Chicago.COM> (Wayne Dockery) writes:
>If the cabin in a commercial passenger aircraft is
>pressurized, how is it possible to exhange fresh
>air from the outside since I would assume the aircraft
>is somewhat airtight?  Also, is the pressure managed
>by a air-compressor or is it simply "scooped" from the
>outside via some sort of duct-work into the cabin thereby
>generating the pressure?

A portion of the air entering the engine is directed to the air conditioning

The air conditioning system, having done its work, releases air into the

The cabin, without a relief valve, will prevent the air from leaving,
thus resulting in an increase in pressure.  Beyond a certain point, the
cabin will break from overpressure.

The *pressurization* system, therefore, is that relief valve.  In this
context, it is called an outflow valve.  There can be one or two per
airplane: the 737 and 727, for instance, only have one.  The valve can
generally be modulated from fully open to fully closed.  It generally
resides near fully closed when at altitude.  The device which controls the
modulation of the outflow valve is called a pressurization controller.
There are typically multiple levels of redundancy.  On the 727, for example,
there are automatic, standby, and two manual modes on an electronic pressur-
ization controller.  There is also a hokey pneumatic controller on older
aircraft, but it hurts to think about it, much less discuss it.

The objective of the pressurization system is to keep the pressure at a
maximum cabin altitude defined by the ambient atmospheric pressure + 8.5

There are also automatic "over-pressure" and "under-pressure" valves, which
help preserve structural integrity.

If you read between the lines in all this, the continuous *inflow* of air
means there must also be a continuous outflow: through the outflow
valve, cracks, etc.  This is how a typical air conditioning system is almost
completely recirculating.

Robert Dorsett                         Moderator, sci.aeronautics.simulation