captainsimon
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737 Fuel Question

Mon Nov 19, 2007 8:51 pm

Is there any reason why the centre tank on this 737 has no fuel in it?
http://www.airliners.net/open.file?id=1294917&size=L
I know this could be a pic where no fuel has been added but it makes me wonder if there is a reason behind it.
 
Dalmd88
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RE: 737 Fuel Question

Mon Nov 19, 2007 9:08 pm

This isn't uncommon. The center tank is only used when needed for longer range flights and is emptied first.
 
AAR90
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RE: 737 Fuel Question

Mon Nov 19, 2007 9:59 pm



Quoting Captainsimon (Thread starter):
it makes me wonder if there is a reason behind it.

The "Main" (wing) tanks are not full. 737 Operating Limitation is that the wing tanks must be completely full PRIOR to putting fuel in the center tank; and center tank fuel MUST be used PRIOR to using Main tank fuel.
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CanadianNorth
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RE: 737 Fuel Question

Tue Nov 20, 2007 1:58 am



Quoting AAR90 (Reply 2):
737 Operating Limitation is that the wing tanks must be completely full PRIOR to putting fuel in the center tank; and center tank fuel MUST be used PRIOR to using Main tank fuel.

The procedure you desribed makes perfect sence, and I think I've heard of this before too. But one thing I haven't heard is the reason why fuel tanks must be filled and emptied in this order. Anyone know why? (i'll try to ask at work tonight, but by the time we finish the Hawkers and go to the jet I'll probably forget about it)



CanadianNorth
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tdscanuck
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RE: 737 Fuel Question

Tue Nov 20, 2007 3:17 am



Quoting CanadianNorth (Reply 3):
But one thing I haven't heard is the reason why fuel tanks must be filled and emptied in this order. Anyone know why?

It reduces fatigue damage to the wings.

The weight of fuel in the wings helps relieve the bending moment due to lift. Fuel in the center increases weight but doesn't provide any bending relief. So, for maximum longevity of the wing, you want to always keep the fuel load as far outboard as you can. Physically, the wing can happily fly with any one tank full and all the others empty (or any combination between that and completely full) but with a concurrent reduction in fatigue life of the wing.

On many designs, there are also oil-fuel heat exchangers in the wing tanks that you won't want to uncover, so you always need some fuel in the wings to keep those systems happy.

Tom.
 
AAR90
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RE: 737 Fuel Question

Tue Nov 20, 2007 6:35 am



Quoting CanadianNorth (Reply 3):
Anyone know why?

Sure. In fact, it was in my original draft reply but deleted as it seemed obvious (to those who fly/work on planes everyday).

Quoting Tdscanuck (Reply 4):
It reduces fatigue damage to the wings.

The weight of fuel in the wings helps relieve the bending moment due to lift. Fuel in the center increases weight but doesn't provide any bending relief. So, for maximum longevity of the wing, you want to always keep the fuel load as far outboard as you can. Physically, the wing can happily fly with any one tank full and all the others empty (or any combination between that and completely full) but with a concurrent reduction in fatigue life of the wing.

Nice explanation. In even simplier terms, the wings provide lift and if the (fuel) weight is in the wings, there is less metal bending. OTOH, if the (fuel) weight was only in the fuselage, the wing structure would need to be significantly beefed up to handle the stress. So.... since you need to carry the fuel anyway, the more fuel weight in the wings means the less fuel weight in the fuselage, and the less structural weight the wing requires. Operationally, most planes require using fuselage tank fuel before wing tank fuel for the same reason.  Smile

Quoting Tdscanuck (Reply 4):
On many designs, there are also oil-fuel heat exchangers in the wing tanks that you won't want to uncover, so you always need some fuel in the wings to keep those systems happy.

For example, the 737NG's.  Smile
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
CanadianNorth
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RE: 737 Fuel Question

Tue Nov 20, 2007 8:53 pm



Quoting Tdscanuck (Reply 4):
It reduces fatigue damage to the wings.

The weight of fuel in the wings helps relieve the bending moment due to lift.

I knew it was probably something really obvious!  silly 

Oh well, thanks for the answers Tdscanuck and AAR90


CanadianNorth
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777236ER
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RE: 737 Fuel Question

Tue Nov 20, 2007 9:05 pm



Quoting Tdscanuck (Reply 4):
It reduces fatigue damage to the wings.

The weight of fuel in the wings helps relieve the bending moment due to lift. Fuel in the center increases weight but doesn't provide any bending relief. So, for maximum longevity of the wing, you want to always keep the fuel load as far outboard as you can. Physically, the wing can happily fly with any one tank full and all the others empty (or any combination between that and completely full) but with a concurrent reduction in fatigue life of the wing.

On many designs, there are also oil-fuel heat exchangers in the wing tanks that you won't want to uncover, so you always need some fuel in the wings to keep those systems happy.

Yes, and you can gravity feed from the wing tanks. I doubt the 737 can suction feed from the centre tank.
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tdscanuck
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RE: 737 Fuel Question

Wed Nov 21, 2007 1:43 am



Quoting 777236ER (Reply 7):
I doubt the 737 can suction feed from the centre tank.

It can't, other than to suck through the center tank boost pumps. The wing suction feed inlets are such an easier path that you'd pull out of them first anyway.

Tom.
 
777236ER
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RE: 737 Fuel Question

Wed Nov 21, 2007 4:09 pm



Quoting Tdscanuck (Reply 8):

It can't, other than to suck through the center tank boost pumps. The wing suction feed inlets are such an easier path that you'd pull out of them first anyway.

To change topic significantly, what do you know about 737 water management? If I can recall there's no dedicated water scavenge jet pumps in any of the tanks (but the centre tank has one scavenge for fuel/water into the wing tanks). Do you know whether water drain mx actions are actually carried out?
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B777Neuss
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RE: 737 Fuel Question

Wed Nov 21, 2007 6:23 pm

And what is about the hazard of the explosive gases in the centre tank like TWA800?
Did Boeing change something after this accident in the layout of the centre tank?
I heard that the nitrogen safety system would be to heavy.
 
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HAWK21M
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RE: 737 Fuel Question

Wed Nov 21, 2007 7:49 pm



Quoting 777236ER (Reply 9):
Do you know whether water drain mx actions are actually carried out

are you reffering to a daily Tank drain schedule.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
tdscanuck
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RE: 737 Fuel Question

Thu Nov 22, 2007 5:00 am



Quoting 777236ER (Reply 9):
To change topic significantly, what do you know about 737 water management? If I can recall there's no dedicated water scavenge jet pumps in any of the tanks (but the centre tank has one scavenge for fuel/water into the wing tanks). Do you know whether water drain mx actions are actually carried out?

737NG has a water scavenge pump in each tank, in addition to the center-to-wing scavenge pump. Having the checkvalve go bad in one of those is one of the hardest fuel issues to troubleshoot that I know of. I can't remember about 737 Classic.

Water drainage (sumping) is strongly recommended, although frequency depends a lot on the operating environment (humidity, fuel supply, etc.).

Quoting B777Neuss (Reply 10):
And what is about the hazard of the explosive gases in the centre tank like TWA800?

There is a looooong FAA NPRM about this. The numbers they use are somewhat in dispute but, no matter which way you slice it, the risk is very very low.

Quoting B777Neuss (Reply 10):
Did Boeing change something after this accident in the layout of the centre tank?
I heard that the nitrogen safety system would be to heavy.

There were a whole bunch of changes as a result of the TWA800 investigation. They're model specific, but they genenerally involve putting in an extra layer of protection all over the place to prevent an inadvertent ignition source in the fuel tank. NGS isn't too heavy...there are 747's and 737's with certified systems flying today.

Tom.
 
777236ER
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RE: 737 Fuel Question

Thu Nov 22, 2007 5:13 pm



Quoting HAWK21M (Reply 11):

are you reffering to a daily Tank drain schedule.

Yep, along with water scavenge pumps etc.

Quoting Tdscanuck (Reply 12):
Water drainage (sumping) is strongly recommended, although frequency depends a lot on the operating environment (humidity, fuel supply, etc.).

For both of you, is water draining done as little on the 737 as it is on other aircraft? How do the drain valves cope with hoarfrost etc?
Your bone's got a little machine
 
tdscanuck
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RE: 737 Fuel Question

Thu Nov 22, 2007 7:30 pm



Quoting 777236ER (Reply 13):
Quoting Tdscanuck (Reply 12):
Water drainage (sumping) is strongly recommended, although frequency depends a lot on the operating environment (humidity, fuel supply, etc.).

For both of you, is water draining done as little on the 737 as it is on other aircraft? How do the drain valves cope with hoarfrost etc?

In my experience, most operators sump less than they should. They tend not to control it by aircraft model, so I would guess it's done the same on the 737 as on other aircraft. And yes, the drain valves can freeze up when it's cold out.

Tom.
 
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HAWK21M
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RE: 737 Fuel Question

Fri Nov 23, 2007 6:40 pm



Quoting 777236ER (Reply 13):
is water draining done as little on the 737 as it is on other aircraft

A Daily Drain Schedule for B737s out here.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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