Re: Cabin pressure controls

From:         bill@texan.rosemount.com (William Hawkins)
Organization: Rosemount, Inc.
Date:         08 Sep 95 02:35:33 
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I've done some research into cabin pressure control as a passenger
with a hiker's altimeter, and I've read Robert Dorsett's articles
on the subject.  I've seen the cabin pressure controller while
riding in the jumpseat of a corporate jet (4 engine, 2 pod,
Lockheed).  My business is process control, not aircraft control
systems.

The cabin pressure controller had both an altitude setting and a
rate of change setting, which was calibrated in feet/min as I
remember it.  There has to be a controller for maximum pressure
difference (inside to outside the hull), but that could have
been as simple as a safety valve.

Pressure control of gas in a closed vessel requires two valves,
one to let gas in and one to let it out.  The combination of
the two can regulate gas flow through the vessel.  In one article
Robert said that the pressure controller regulated the outlet
valve position.  Something still needs to regulate the amount of
air bled from the engines.

Pressure is under the control of the flight deck, up to a point.
Climbing out of Amsterdam on a 747, my altimeter kept reading
sea level for 5 minutes, as the ground dwindled away below us.
Then it began rising at about 300 feet per minute, leveling off
at about 6000 feet.  This was held during cruise, but rose to
8000 feet when the engines were throttled back for descent.  This
makes sense, because the engines are not producing enough bypass
air to maintain pressure.  There's nothing the Captain can do
about that.  The worst time I have with pressure is on approach,
when the engines spool up and down as required to stay on the
approach path.

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.  On
one flight, a child adross from us became violently airsick.  We
had no option to change seats on a computer optimised flight load.
I said to a flight attendant, "You know how you ask if there's
anything at all we can do to make your flight more pleasant, well
there is."  "Change your seats?" she says.  "No, ask the flight
deck to increase the ventilation in here to what it was when you
carried a load of smokers."  She went away shaking her head.

So, what are the cabin air control systems comprised of, in terms
of sensors, controllers, and actuators?

Bill Hawkins  bill@bvc.frco.com