Re: Power Hungry air conditioning ?

From:         barr@netcom.com (Keith Barr)
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
Date:         17 Mar 95 12:58:03 
References:   1
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In article <airliners.1995.285@ohare.chicago.com>,
Chris Wesley         (0013 cxw ) <chris.wesley@tiuk.ti.com> wrote:
>Does the air conditioning consume a significant portion of the engine power ?
>I thought it was driven from the APU anyway.

Yes, A/C does take a noticible portion of engine power.  In normal flight a
small portion of compressed air is taken from the compressor section of the
engine.  This air is then run through a precooler, and then piped on to the
PACKs (Pneumatic Air Conditioning Kits), where the still hot air is mixed
with an appropriate amount of cool air and then it is pumped into the
cabin.  Using engine bleed air reduces the amount of thrust available,
so EPR corrections are shown in the flight manual.

The before takeoff checklist on the 737-200 (I assume other types are
similar) call for turning the left and right PACKs on, engine #1 and #2
bleeds off, APU bleed on, and closing the isolation valve.  The isolation 
valve simply prevents the bleed air from the APU from going to both the left 
and right PACKs.  When closed only the left PACK sees APU bleed air.

Climbing through 1000' above ground level, the pilot flying asks for the
after takeoff checklist which calls for turning on #1 and #2 bleeds, moving
the isolation valve to auto, and turning off the APU (at pilots discretion).

Fun Story:  Last September I was on a research flight on a Convair 580, and
the Pressurization/Heating/Cooling system broke in mid flight.  While air
was still being pumped in to maintain pressure, the air was not being
cooled.  Even though we started an immediate descent from FL240, by the
time we got down to 10,000 MSL, where we could turn off the pressurization
completely, the cabin temperature was a balmy 125F!  It actually felt good
to get off the plane and get into the reasonable 100F temperatures of Palm
Springs, CA. :^)

BTW: You can see a picture of this exact Convair on page 33 of the March 13th 
Aviation Week & Space Technology.
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