Boeing 707 Engine Pylons

From: (Richard Keeling)
Organization: Adacel Pty Ltd, Melbourne, Australia
Date:         17 Dec 95 14:06:23 
References:   1
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The engine pylons on the Boeing 707 housed the cabin air conditioning units.
Passenger versions of the B707 had three engine driven air-cycle
air-conditioning units in engines 2,3 and 4, the 320C freighter had only two,
engines 3 and 4. These units achieved a cabin differentional of 8.6 lbs/sq in.
 The smaller B720 only required two units as per the freighter versions.  The
B720 was almost externally identical to the heavier B707s but had a different
structure and corresponding lighter operating weights.

 cheers  Richard Keeling.