747 "smoke" trail

Date:         19 Apr 99 02:22:20 
From:         Frank Jenkins <fjenkins@Savantics-Inc.com>
Organization: Savantics Inc.
Followups:    1
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A couple of days ago, I was looking at a 747-400 pass over my house at
about 5000 feet on landing vectors to SFO.  While watching, I noticed a
white stream of smoke or mist starting to come from the tailcone area.
This continued for a short period of time, leaving a 10-15
airplane-length trail in the sky.  I assume this smoke had something to
do with the APU.  However, I have watched a *lot* of jets and never seen
this before.

Does anyone have a possible explanation?

Frank Jenkins