Why does the b747-400 have a fuel tank in the horizontal stabilizer?

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Request for Question Clarification by tutuzdad-ga on 18 Nov 2004 18:36 PST

Are you asking "What is the purpose of the tank located on the
horizontal stabilizer?" or are you asking "Why was the decision made
to place the tank on the horizontal stabilizer?"

tutuzdad-ga

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Clarification of Question by dcmilpilot-ga on 18 Nov 2004 20:19 PST

The later.  I'm content in knowing that any placement of additional
fuel will be benificial to aircraft range/preformance.

I am asking...Is there an aerodynamic advantage to the placement in
the Horizontal Tailplane?  Was Boeing going for added rear CG,
additional stability, something else?  As opposed to the addition of
Aux tanks in Cargo holds, the belly, etc.

Thanks.

> Was Boeing going for added rear CG <

DCMilpilot --

You do realize that the rear-most CG in the allowable design envelope
is the "most efficient" flight profile?

My former CFI, an American Airlines pilot, tells this story: 
"We had a program at American in which employees could suggest an
efficiency move and get paid a percentage of the savings, should it be
adopted.  Then a pilot suggested that we fly every plane with an aft
CG, a suggestion that would save millions of dollars.  But it kind of
broke the suggestion program."

Here's a Boeing page on the effect of aft-CG on efficiency:
http://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html

As Boeing says:
"Flying at an aft CG will reduce the download on the tail and improve
cruise performance.

When airplanes are operated near the aft CG limit, download on the
tail is minimized and angles of attack and drag are reduced. However,
moving the CG aft reduces the longitudinal static stability of the
airplane, something that all flight crews should be aware of."

Of course that "longitudinal static stability" means a bunch of things to a pilot:
*  more likely to stall or spin
*  more pilot effort during flight
*  careful attention to what happens to the CG as fuel burns off. 
(I'd bet that the 747-400 manual calls for the tail tanks to be used
first.)
*  weight shifts could make the aircraft uncontrollable

Best regards,

Omnivorous-GA