Jack Davis wrote:

On Wed, 30 Mar 2005 04:09:15 GMT, Gord Beaman
wrote:

(Roy Smith) wrote:

I was under the impression that most big jets had a small fuel tank in
the tail, and they could pump fuel back and forth to trim the CG. Am
I mistaken?

No you're not but they use it for economy...most a/c are built to
be quite nose heavy so they need quite a lot of nose up trim in
level flight, this increases the 'fore and aft stability'.

Huh? I've not flown a jet with a "small fuel tank in the tail". Am I
missing something?

-Jack Davis
B737

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.

This system is only used in stable cruise. IIRC this system
caused the crash of a large Russian airliner which was handled
roughly while in this mode by the captain's son.
--


-Gord.

"I'm trying to get as old as I can,
and it must be working 'cause I'm
the oldest now that I've ever been"