Re: Power Hungry air conditioning ?

From:         jimh@eskimo.com (Jim Hogan)
Organization: Eskimo North (206) For-Ever
Date:         17 Mar 95 12:58:03 
References:   1
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In article <airliners.1995.285@ohare.Chicago.COM>, 
crazy!a904246@uunet.uu.net says...

>I was taking off from Mombasa last Monday and was surprised to hear the pilot
>advice us that - due to the high outside temperature causing lower engine
>efficiency - he would be turning off the cabin air conditioning during takeoff
>to get more power.

>Does the air conditioning consume a significant portion of the engine power ?

The air conditioning packs are air cycle machines (see any good thermo 
book for an explanation) which use air bled from the core of the engine 
both for their power (by means of an inlet turbine) and as the operating 
medium (i.e. the air coming out of the engine is what eventually winds up 
in the cabin at the desired temperature).  No bleed takeoffs are 
typically performed at high altitude and/or temperature airports where 
the pilot does not want to give up the thrust lost to the A.C. packs.  

>I thought it was driven from the APU anyway.

The APU can also be used as a bleed air source for the packs.  The other 
alternative is to hook the airplane to a ground pnuematic cart.

Jim