Why are center tanks not used?

tom418 writes:

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

I just meant that the wing tanks usually contain fuel and are filled
with equal amounts of fuel, up to and including all that is necessary
for the flight. The center tank apparently remains empty, unless its
extra capacity is needed, even to the point of not turning on the
center tank pumps (I think). And supposedly the manufacturer forbids
putting more than a small amount of fuel in the center tank (at least
in the 737) without adding at least as much to the wing tanks.

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".

Hmm. Sounds quite different from the 737. I wonder why.

I've always read that the 727 is a great aircraft to fly. It
certainly made history. The 737 is now doing the same.

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