Re: TWA800 Fuel Tank

Date:         01 Dec 97 02:33:52 
From:         David Tyler <tyler@plk.af.mil>
References:   1
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Gerard Foley wrote:
> To reduce the probability of a fuselage mounted fuel tank exploding,
> as did the tank in the 747 on TWA Flight 800, why not fill such a tank
> first and empty it last.   Very few airliners exhaust their fuel reserves
> in flight.

fuel loading and burning sequences are determined by two factors:
1.) center of gravity (CG) , and 2.) centerline weight.  in a swept-
wing aircraft like the 747, fuel in the wing tanks moves the CG aft,
while fuel in the center tank moves in forward.  i'm not now and never
have been qualified in the 747, but in the 707, you couldn't keep an
arbitrarily large amount of fuel in the center tank without a good
deal of fuel in the wings-- the CG would be out-of-limits forward.
also, the wings have a ``center of lift;" a location along the wing
where the upward force of lift is centered:
         __
           |  ____________________
  Fuselage  | wing  ________------|
            | ------
         __|        ^
             X      |         Y
                center of lift

please forgive the ascii ``art."

now, consider that if you have a _lot_ of weight in the fuselage
(like lots of center tank fuel in addition to payload) and _very
little wing fuel_, the lift generated by the wing tends to _bend
the wing_ at the point i've marked `X.'  this is not good for the
service life of the aircraft!  however, if you have wing fuel weight-
ing the wing down at the point i've marked `Y,' the weight keeps
lift from bending the wing about `X.'  ahh...  much better.

there's one final reason the practice you've suggested is not foll-
owed:  wing tank fuel can gravity-feed to the engines without any
electrical power required to pump it.  you can see that if you took
off on a long overwater burning centerline fuel first, then exper-
ienced something really fun like loss of electrical power to the
bus powering the center tank pumps, well, you may a.) cuss a lot and
turn around if you havn't burned much center fuel because you know
you need it to get to your destination and now you can't have it, or
b.) press on with wing fuel if you've burned most of the center gas
prior to the pump failure.  if you take off and burn the wing fuel
first, getting way out over the ocean, then start burning the center-
line fuel and _then_ have the failure-- well, you are not having a
very good day at the office.  all that centerline gas may as well be
water for all the good it will do you!  cheers,

dave
______________________________________________________________________
-David W. Tyler               "It seems you feel our work is not
-USAF Phillips Laboratory         of benefit to the public."
-Albuquerque, New Mexico
-tyler@plk.af.mil                         --Rachel