Re: Cabin pressure

From:         eemaan@aimnet.com (Ethan Schell)
Organization: None
Date:         01 Jun 95 05:00:59 
References:   1
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In article <airliners.1995.686@ohare.Chicago.COM>
wdockery@pipeline.com (Wayne Dockery) writes:

> 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?

The aircraft is somewhat airtight, except for one or more outflow
valves.  These valves are small doors measuring about one foot square
that have an actuator that opens or closes the door at the command of a
pressure controller.  The pressure controller opens or closes the
valves to regulate cabin altitude and rate of climb/descent.  The
pressurization supply is the air conditioning system which pumps
outside air into the cabin through various vents, including the eyeball
vents above your seat.  Now, pressurization is accomplished by letting
less air out through the outflow valves than is being pumped in by the
air conditioning system until the desired pressure is attained; then
this pressure maintained by equalizing inflow/outflow.  The pressure
controller is pretty smart - even if the pilot expedites descent at
4000 feet per minute, your ears will remain comfortable at a 500 foot
per minute descent.