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Usage Of Center Tanks On B737  
User currently offlineA300605R From Germany, joined Nov 2005, 136 posts, RR: 0
Posted (7 years 8 months 1 week 6 days 18 hours ago) and read 6602 times:

Just came across this photo of the flightdeck of a 738 and wondered why the wing tanks are full whereas the center tank is almost empty.

I know that an aircraft will never be fueled completely for short hops.

But is it written in the user manual to use the wing tanks preferably or is it up to the Capt. to decide which tanks are used? What's the reason behind this?
The fuel pumps?

http://www.comlaw.gov.au/comlaw/Legi...016C/$file/B737-198%20Amdt%201.pdf
"The United States Federal Aviation Administration (FAA) has received reports
indicating that fuel pumps on Boeing Model 737, 747 and 757 series aeroplanes have
failed as a result of chafing of the stator lead wire bundle. This occurred when the
stationary wire bundle came into contact with the rotor in the pump motor. The
pumps failed when the pump power was short-circuited to the rotor and the circuit
protection device tripped.
[...]
The centre tank fuel pumps must be OFF for takeoff if centre tank fuel is less than 5,000 pounds (2,300
kilograms) with the airplane readied for initial taxi.
Both centre tank fuel pump switches must be selected OFF when centre tank fuel quantity reaches
approximately 1,000 pounds (500 kilograms) during climb and cruise or 3,000 pounds (1,400
kilograms) during descent and landing. The fuel pumps must be positioned OFF at the first indication
of fuel pump low pressure."

Any input is welcome!
Thanks!
 wave 


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User currently offlineAAR90 From United States of America, joined Jan 2000, 3471 posts, RR: 47
Reply 1, posted (7 years 8 months 1 week 6 days 17 hours ago) and read 6594 times:

The 738 operating manual requires Main (wing) tanks to be filled completely prior to putting any fuel in the Center (fuselage) tank. If there is fuel in the center tank, it is required to be emptied PRIOR to using Main (wing) tank fuel. This is for weight/balance and structural limitations. The FAA useage mandate simply defines the ("new" and very) specific procedures for when Center tank fuel pumps must be "on" and when they must be "off."


*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineA300605R From Germany, joined Nov 2005, 136 posts, RR: 0
Reply 2, posted (7 years 8 months 1 week 6 days 17 hours ago) and read 6588 times:

Quoting AAR90 (Reply 1):

Ah, ok! Thanks a lot!
When I start thinking about why an aircraft has wings and what happens there, it becomes clear...  Wink



300 319 320 321 332 733 734 735 738 753 763 F27 M83
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 3, posted (7 years 8 months 1 week 6 days 13 hours ago) and read 6509 times:

The original intent of the design was that it was treated, in NORMAL operation, almost as if it was a single tank. Functionally very simple.

You topped the mains first, ten thousand pounds in each, and any remaining of the required fuel would go in the center tank. It burned center first, then mains. When fuel was in all three tanks and all pump switches were on the center tank pumps would overpower the left and right wing tank systems (it would seat the one-way check valves) and both engines would feed entirely from the center tanks until that fuel was exhausted.

When the center tank was empty the associated fuel LOW PRESSURE lights would come on. Turning the pump switches OFF disarmed these lights. So it was common practice to leave these off for takeoff unless there was enough fuel in that tank to keep the uptakes covered and prevent nuisance low pressure lights on takeoff.

After the center pumps were turned off pressure from the left tank pump system would operate a scavenge pump in the center tank and transfer the remaining fuel to tank #1. This shut off automatically after twenty minutes.

After TWA 800 and some other events they modified the pump usage to what is reference in reply #1. I have not flown the B-737 since that time and cannot comment on new procedures.

Center tank fuel has to be considered almost as part of "zero-fuel weight" for structural purposes, as it is located between the main gear and causes considerable bending moments in the wing spar when on the ground. This is offset by wing tank fuel which is located outboard of the mains.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 4, posted (7 years 8 months 1 week 6 days 3 hours ago) and read 6441 times:

Structural Point of view.Fill Wing tanks before Filling Ctr tanks.Also Ctr tanks fuel is consumed first as the Boost pump check valve of th Ctr tank opens at a lower setting than the one in the Wings.
regds
MEL



Think of the brighter side!
User currently offlineA300605R From Germany, joined Nov 2005, 136 posts, RR: 0
Reply 5, posted (7 years 8 months 1 week 6 days 1 hour ago) and read 6428 times:

Quoting SlamClick (Reply 3):



Quoting HAWK21M (Reply 4):

Very interesting information! Thank you!  bigthumbsup 



300 319 320 321 332 733 734 735 738 753 763 F27 M83
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 6, posted (7 years 8 months 1 week 6 days 1 hour ago) and read 6424 times:

Just to add that this is common to all aircraft (that I know of.)
Fuel is kept in the wings as long as possible, to provide bending relief. The generation of lift pulls the wings up, and the weight of fuel pulls them down to put it simply.
The B757 767 and 777 are exactly the same as the B737.
The Tristar, B747 and A319/20 (not A321) also have outboard wing tanks which are kept full of fuel as the inboard tanks are used.

By the way to increase the effect, on the B737NG the wing tanks are made smaller than on the -400, and some centre tank fuel is in the wings.

Note that on the ground during maintenance and fuel service you can do what you want. If we need to work in the right wing tank, we can transfer all the fuel to the centre tank, or right wing tank. Likewise during refuelling, you can fill all tanks at the same time.


User currently offlineNonfirm From United States of America, joined Sep 2005, 434 posts, RR: 0
Reply 7, posted (7 years 8 months 1 week 6 days ago) and read 6409 times:

Quoting HAWK21M (Reply 4):
Structural Point of view.Fill Wing tanks before Filling Ctr tanks.Also Ctr tanks fuel is consumed first as the Boost pump check valve of th Ctr tank opens at a lower setting than the one in the Wings.

On our flight that require center fuel all of the tanks are filled at the same time.Also i do not think the check valves are different.The center pumps have an output of 23 psi.The output for the wing pumps is at 10 psi.The low pressure lights for the center come on at 22 psi or less with a master caution warning in 10 sec and on our 800's after 15 seconds a relay will shut power off to the left center pump.The control panel is p-61 i think.The low pressure lights for the wings will come on at 6 psi.  airplane 


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 8, posted (7 years 8 months 1 week 5 days 23 hours ago) and read 6389 times:

Quoting TristarSteve (Reply 6):
Just to add that this is common to all aircraft (that I know of.)
Fuel is kept in the wings as long as possible, to provide bending relief. The generation of lift pulls the wings up, and the weight of fuel pulls them down to put it simply.
The B757 767 and 777 are exactly the same as the B737.
The Tristar, B747 and A319/20 (not A321) also have outboard wing tanks which are kept full of fuel as the inboard tanks are used.

This is the important point. It's simply a manifestation of gravity. Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 9, posted (7 years 8 months 1 week 5 days 20 hours ago) and read 6380 times:

Quoting SlamClick (Reply 3):
Center tank fuel has to be considered almost as part of "zero-fuel weight" for structural purposes, as it is located between the main gear and causes considerable bending moments in the wing spar when on the ground.

I realize that there's an "almost" in your statement, but to ensure that nobody gets the wrong idea, the only time center tank fuel has to be considered a part of the ZFW is when fuel is potentially/actually trapped in there due to inop/MEL'ed center tank fuel boost pump(s).


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (7 years 8 months 1 week 5 days 19 hours ago) and read 6367 times:

Quoting OPNLguy (Reply 9):

You are correct sir, and I hope everybody noticed the "almost" in there.

The "maximum zero-fuel weight" that is part of the limitations on the FAA type certificate is predicated on the fuel being loaded and burned off in standard manner.

For those who don't know, "zero-fuel weight" is the sum of the operational empty weight of the airplane, the crew, the passengers, their luggage, cargo and galley supplies. In short, everything but fuel. In the real world the airplane would never be in this actual condition but it must be protected in loading.

So picture a plane at the gate with only a couple thousand pounds in each wing tank, getting payload added right up to Max ZFW plus the sum of the fuel in the wings. Now if you were going to put a lot of fuel aboard, and if you started filling the center tank first you could put more unbalanced weight in the center, between the main gear than the designers intended. In other words if they thought you were going to do this they'd have set Max ZFW lower.

On the other hand when you start by filling the wings you are balancing the loads across the "fulcrum" of the main gear.

For weight and balance purposes however, the fuel in this discussion was never part of the Zero-Fuel Weight.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 11, posted (7 years 8 months 1 week 5 days 19 hours ago) and read 6364 times:

Quoting TristarSteve (Reply 6):
By the way to increase the effect, on the B737NG the wing tanks are made smaller than on the -400, and some centre tank fuel is in the wings.

Just to clarify, the wings pass through a notch in the fuselage structure. They can be identified as separate components as they pass through. If you stand in the wheel wells looking forward you are looking at the rear spar which is also the aft bulkhead of the fuel cells.

I am not familiar with the 737 -100, -200, or NG but I know the -300 and -400 pretty well. On the 300 and 400 the center "tank" is within the wing structure as it passes beneath the deck of the fuselage. Transport category does not permit fuel to be stored in the fuselage. The outer ends of the center tank are pretty well outboard, near the maingear mounts, in fact. Many components shown in a schematic are not physically located where they are illustrated. For example most B-737 fuel system schematics would show the fuel crossfeed valve to be forward of the tank, possibly toward the left side. In reality it is in the right main gear well.

Nominal capacity for a -300 might be ten thousand pounds in tanks 1 and 3 and 15,500 lbs in the center tank. Being half-again larger than the main tanks and giving up some space for landing gear wells and A/C Pack area, they do extend beyond the gear, but the fuel weight is considered to act at a centroid which would be pretty well on centerline.

Also, you can service the wing tanks through overwing caps but to less than full capacity. You cannot service the center tank in this fashion. If you had to get fuel into that tank you could possibly (company permitting) transfer on the ground.

This would be done by turning on the wing tank boost pumps, opening the fuel crossfeed and the defuel valve, then opening the center tank fill valve. It would take a while.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 8 months 1 week 5 days 18 hours ago) and read 6345 times:

Quoting SlamClick (Reply 11):
Transport category does not permit fuel to be stored in the fuselage.

One of my previous airlines flew the Shorts SD-330, and I never did feel really comfortable knowing that there was a fuel tank above the cabin on this type (and presumably, the single-tail SD-360). I think I read somewhere else here on Anet that the USAF had similar concerns about this.

Quoting SlamClick (Reply 11):
Also, you can service the wing tanks through overwing caps but to less than full capacity.

The NG-family brought several changes to the fuel system, and eliminating over-wing fueling capability was one of them. Someone already mentioned the thinner wing (NG wings hold about 17,200 lbs. versus the previous 19,000-something), and that thinner wing brought a new problem along with it. If you're tankering fuel on long flights, the thinner wing gets sufficiently cold-soaked where landing at an airport with high humidity can later produce ice on the wings. It was a bit odd to see a de-icing delay when SAN or FLL was 70F outside. We quit tankering on some flights 2:30 or more in duration, and that seemed to eliminate the problem.

Still, that's a minor quibble. I've dispatched the -100, -200, -300, -500, and -700 in my career, and the -700 is definitely the best of the bunch as far as performance, and in my book, it's a mini-757. True, it's more an "electric-jet" than the -200s were, but the NG's many pros outweigh the potential cons. The last airline I worked at had the 737-200ADV with -17 engines, but in 1982 when they were delivered, it seemed pretty clear that it was the end-of-the line as far as JT8D development (for the 737) went, and that the latest versions of the high-bypass engines on the then-new 757s and 767s would soon find their way onto new versions of the 737.


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 13, posted (7 years 8 months 1 week 5 days 18 hours ago) and read 6331 times:

Quoting OPNLguy (Reply 12):
the -700 is definitely the best of the bunch as far as performance

I built some PowerPoint courseware for teaching -700 performance and the big problem with teaching performance on that airplane is that there IS NO problem. The answer is "yes it can do that" almost every time you ask the question.

For example, the 737-700 can land at RNO on the hottest day in Reno's history, at maximum certificated landing gross weight, do a quick turn and take off at maximum certificated takeoff gross weight.

Now how do you teach performance limitations with an airplane like that? Invent LaPaz Bolivia non-stop to Deadwood Colorado legs?

Quoting OPNLguy (Reply 12):
737-200ADV with -17 engines

In some respects that is about as good as the 737 needed to get (especially the -AR engines) with the original wing. The wing and tail are the big differences in the NGs.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 14, posted (7 years 8 months 1 week 5 days 18 hours ago) and read 6327 times:

Quoting OPNLguy (Reply 12):
the Shorts SD-330, and I never did feel really comfortable knowing that there was a fuel tank above the cabin on this type (and presumably, the single-tail SD-360). I think I read somewhere else here on Anet that the USAF had similar concerns about this.

I don't know about the USAF but I do occasionally see a C-23 Sherpa here, so the Army eventually bought off on the idea.

Army Special Forces took a good look at the Shorts Skyvan back in the 60s and while I did not get the opportunity to fly it, the consensus was that no one wanted to ride into a "hot" landing area with fuel above the cargo (troop) area.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 15, posted (7 years 8 months 1 week 5 days 17 hours ago) and read 6316 times:

Quoting SlamClick (Reply 11):
On the 300 and 400 the center "tank" is within the wing structure as it passes beneath the deck of the fuselage. Transport category does not permit fuel to be stored in the fuselage.

On a -400, when you take up the cabin floor are you looking at the fuel tank? Or are there two sheets of structure there with a space in between?
I am pretty certain that on a -200, you see the top of the fuel tank under the cabin floor, but I wait to be fired down as it is a long time ago I was in there.
Perhaps the upper wall of the centre tank, being the pressure face, the fuel tank is considered to be outside the fuselage.

Quoting OPNLguy (Reply 12):
One of my previous airlines flew the Shorts SD-330

My next door neighbour in BAH was the only BAH based Skyvan driver in GF. (The others were down in MCT.) So he had to do the idle power rate of descent checks after engine work.He always came in the hangar looking for volunteers to go up as observer, but never got any takers!


User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 16, posted (7 years 8 months 1 week 5 days 17 hours ago) and read 6316 times:

Quoting SlamClick (Reply 13):
Now how do you teach performance limitations with an airplane like that? Invent LaPaz Bolivia non-stop to Deadwood Colorado legs?

I know the feeling. When I was an ADX designated examiner, I had 6 scenario 727 flights for use on the practical, and converting them to 737 use "solved" many of the problems I had built into the scenarios....  Wink

Those -200ADVs with the -17s were indeed great airplanes; 124.5 MTOW and 107.0 MLW, and problems were few and far between. The only disadvantage to them now would be their fuel consumption compared with newer CFM-powered versions of the 737. Air Florida only had three of them, and when they went under, SWA got one, and we ran it de-rated to -9A, and we eventually hung -9As on it and disposed of the -17s at a profit. The other two went to Midway (v1.0) and although they evenually came to us some years later, I can't recall if we ran them de-rated and later replaced the engines, or the engines were swapped out before being turned over to us. In any event, in their original configuration, they were great birds...


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 17, posted (7 years 8 months 1 week 5 days 16 hours ago) and read 6302 times:

Quoting SlamClick (Reply 13):
Now how do you teach performance limitations with an airplane like that? Invent LaPaz Bolivia non-stop to Deadwood Colorado legs?

Don't tell them over in gen_av or they'll start debating the viability.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineOPNLguy From , joined Dec 1969, posts, RR:
Reply 18, posted (7 years 8 months 1 week 5 days 16 hours ago) and read 6299 times:

Quoting TristarSteve (Reply 15):
He always came in the hangar looking for volunteers to go up as observer, but never got any takers!

Gee, I wonder why...?  scratchchin 

The Shorts SD-330 also had the distinction (in my book) of having the most uncomfortable cockpit jumpseat on the planet, and it was also particularly dangerous to those of the male persuasion if the captain moved his seat aft without warning....


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 19, posted (7 years 8 months 1 week 5 days 16 hours ago) and read 6297 times:

Quoting TristarSteve (Reply 15):
Or are there two sheets of structure there with a space in between?

I believe this to be the case.

I am not a mech, and it has been a long time since I crawled around a 737 in heavy mx but I'm almost certain of it. There is no reason for there being very much dead space there, but the wing center section is a wing. It is built like a wing, shaped like a wing and so forth.

On the other hand the fuselage is a roundish tube until it comes to the wing carry-through area. The pressure vessel, the pressurized part of the tube include the forward and aft bag pits but not the wheelwells. Therefore, there would be a pressure bulkhead at the rear end of the forward pit and the forward end of the rear pit and a cabin floor strong enough to contain the pressure differential from one station to the other. It seems that it would be easier, simpler and cheaper to build this as part of the tube than to rely on plugging a hole with a section of upper (center) wing surface.

My tech library does not include structural documents and I could stand to be corrected on this. FAR 25.963 says in part:
(d) Fuel tanks within the fuselage contour must be able to resist rupture and to retain fuel, under the inertia forces prescribed for the emergency landing conditions in §25.561. In addition, these tanks must be in a protected position so that exposure of the tanks to scraping action with the ground is unlikely.


In general, fuel lines that must pass through the fuselage contour tend to be routed through drain shrouds, scuppers, vented outside the airplane. It just makes sense to me that it would be built as I describe with wing and cabin being two entirely separate structures, joined by hardpoints.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 20, posted (7 years 8 months 1 week 5 days 16 hours ago) and read 6294 times:

Quoting SlamClick (Reply 19):
In general, fuel lines that must pass through the fuselage contour tend to be routed through drain shrouds, scuppers, vented outside the airplane

Yes , I changed the APU fuel feed line once on a B737-200. It is a single piece rubberish hose in one piece, which runs inside a metal tube which is drained to the drain mast beside the left wheel bay.


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