Andrew Sarangan writes:

No one places the loads to exactly so as to balance the airplane. The
elevator downward force is increased or decreased during flight to
balance the airplane for varying load conditions. For example, when a
passenger walks forward, the elevator downward force must be increased
to counteract that. The CG is calculated using the moments and arms to
make sure that it falls within the approved limits. If the CG falls too
far forward or too far backward, the elevator may not be able to
provide the force necessary to balance the airplane.
See he http://en.wikipedia.org/wiki/Center_...%28aircraft%29

I didn't realize Wikipedia had an article on this. I'll take a look.

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