Gord Beaman wrote:

Only on larger a/c Jack...747 have them (some at least). What I
find interesting is how few people seem to know much about them,
is it that they aren't used much now?...that they've gone out of
style?...that the risks of using them isn't worth the
savings?...what?. They're called 'Trim Tanks' I believe and the
theory is to replace the aerodynamically derived 'down force' of
the tailplane with fuel weight at some expense of fore/aft
stability which is compensated for by use of very capable
autopilots.

The Concorde used trim tanks. There's a fascinating explanation of it at
http://www.aircraft-info.net/aircraf...iale/concorde/.
Sounds like pumping fuel around to maintain proper CG during different
flight conditions was a full time job.