Why are center tanks not used?

When you say "filled" do you mean amount of fuel in the each tank compared
to each tank's capacity?

On a B727, there is a limitation which calls for tanks 1 and 3 to contain no
more than 2000 lbs than tank 2 (the center tank), prior to takeoff. So, if
you fill tanks 1 and 3 (about 12,000 lbs) you would have at least 10,000 lbs
in tank 2 (capacity of about 30,000 lbs.) to comply with this limitation.-
Tank 2 would be "partially filled" compared to its capacity, but hardly
"empty".
"Mxsmanic" wrote in message
...
Why does it seem that wing tanks are filled in commercial airliners in
preference to center tanks? What is the advantage to filling the wing
tanks while keeping the center tank empty or partially filled?
Wouldn't heavy wing tanks increase the inertia of the aircraft around
the roll axis?

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