New MD-80, MD-88 tailcones?

From:         papadopo@cs.utk.edu (Philip Papadopoulos)
Date:         11 Aug 93 02:48:29 PDT
Organization: CS Department, University of Tennessee, Knoxville
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I noticed on a recent trip that some MD-80's (MD-88's, too) have
"flattened" tailcones (versus conical ones). Could someone give
me some history on this development. It looks like it is
a retrofit product on older MD-80's.  Can you find
out why the new design tailcone was adopted (fuel efficiency,
stability?) How much does it cost to refit an older plane and
what are the monetary benefits? When was the "new" tailcone
introduced. 

BTW, the flattened and elongated tailcone is very apparent on
American jets, less so on Delta Jets. I saw about 8 Northwest
MD-8x's and none had the flattened cone. All the American Jets
I saw did have the new cone (about 15 examples) and most
(but not all) of the Delta jets had flattened tailcones.

Thanks,
Phil