Re: Cabin pressure controls

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         10 Sep 95 13:26:29 
References:   1 2 3
Followups:    1
Next article
View raw article
  or MIME structure

In article <airliners.1995.1409@ohare.Chicago.COM> (William Hawkins) writes:
>valve position.  Something still needs to regulate the amount of
>air bled from the engines.

On designs I've examined, there really isn't anything.  Engine bleed goes
to the packs, which go through the air distribution system, which results
in the pressure build-up in the cabin, which is then regulated by the
outflow valve, which is regulated by a motor, which is (finally) regulated
by the pressurization controller.

There can be variations of pressure (and certainly variations of rate of
climb) depending on engine thrust settings (and hence airflow).  For example,
if you lose an engine on a typical leaky 727 at altitude, you'd have to start
descending pretty soon in order to maintain pressure.

On the 727, the distribution system has one flow buffer to control the
amount of airflow to the flight deck.  This is a fixed proportion (15-20%
of the total air).  There is also a overpressure valve, to protect the
distribution system itself.  This is not part of the pressurization system.

>I've no idea how flow is controlled.  I hear it is lower now that
>there is less smoking aboard flights, but I don't know if it can
>be controlled from the cockpit or must be set on the ground.

The  only flow control available to the crew is to shut off a pack.  On
the 727, there are two packs: if you shut them off, the airplane will grad-
ually depressurize.  Since pack operation robs you of a couple of EPR points,
this results in higher operating costs.  So some airlines encourage the
deactivation of packs.

There is also an "auto pack trip" option at takeoff, which will trip the
packs if there is engine failure at takeoff.  This helps provide maximum
possible engine power.

On older 727s, there was a little airflow lever, to further control bleed
effects.  For example, you'd want maximum airflow while boarding passengers
(all the hot bodies).  But this was not part of the overall pressurization

Robert Dorsett                         Moderator, sci.aeronautics.simulation