B737-800 fuel tanks

[kilo]

I heard that there is some fuel can not be used under low fuel condition due to tank structual limit,is this true?And,what is the
exact quantity?

Thanks in advance!

[Sperm Bank]

Kilo there is no structural limit for the fuel tanks. The limit you have probably heard about is a Boeing limitation of 450 - 500 kg to remain in the centre tank. This is due to the fuel pumps currently in use, and they don't want us to operate them without being immersed in fuel. That is a very basic answer but hope it helps.

[kilo]

Thank you any way,sperm bank!You seem a little bit confused,

I'm just wonderring how much unuseble fuel will remain in the

main tanks if we try to burn off all fuel.

[Sperm Bank]

Sorry mate I will try again. All of the centre tank fuel is used. There is no unusable fuel in the centre tank. As you are aware, when the fwd No. 1 fuel switch is on, the centre tank fuel scavenge jet pump operates automatically to transfer ANY remaining fuel to the No.1 tank. Transfer begins when the No. 1 tank is about half empty. Once the scavenge process begins it continues for the remainder of the flight whether there is fuel in there or not.

[eng123]

There is always a small amount of fuel that is unuseable in the tank as the suction inlets to the boost pump fuel lines are situated an inch or so above the bottom of the tank.However you must also remember the hydraulic fluid heat exchangers are in the wing tanks and if you run the tanks below 750kgs there is insufficient fuel left to cool the hyd fluid.

[mono]

I am sure the NG is the same as all other a/c, in that any fuel considered unuseable must NOT be included/displayed as a fuel quantity.

Therefore, when you are down to your unuseable fuel level the fuel gauges will read zero. The actual quantity is arbitary 'cos there will be no more fuel going to the engines and your NG will become a very expensive glider.:}

[Biggles89]

There is an alternate procedure from boeing were you only turn off one centerpump when fuel is below 1000kg in center and let the other run until first flicker of fuel+mastercaution. This will normallt happen when you have almost no fuel left in the centertank.


:ok:

[kilo]

By Mono:

I am sure the NG is the same as all other a/c, in that any fuel considered unuseable must NOT be included/displayed as a fuel quantity


Mono,can you find any documents to support your opinion?

[Flap Sup]

Mono/Kilo

Unusable, trapped or ballast fuel will always be included in the gauges, as far as I know. Please remember that unusable fuel is a non standard situation, fuelgauges do not take that into account.

However it will not be on the flightplan as fuel, but as ballast, or other kind of nonrevenue load. Same goes for loadsheet.

For NG drivers: Did it require any change for allowing the use of all fuel in the centertank, or was it just certification issues?

FS

[avioniker]

As long as we're quoting manuals;
The Fuel Quantity indication system shows total fuel quantity with differing annunciations regarding fuel level (i.e. usable, configuration, overfill, etc.) The gauges do not "hide" unusable fuel. The original intention was that the CONFIG message would show and the indication would turn yellow if there was usable fuel in the center tank with the pumps turned off. This occured with 1600 or more pounds in the tank. With the AD's that have been issued the wording has changed but the indication remains the same. A lot of people still think the system doesn't show unusable fuel. That's not the case.

From the 737-800 SDS

28-41-00-006
"FUEL INDICATING - PROCESSOR - FUNCTIONAL DESCRIPTION

General
The fuel quantity processor unit (FQPU) has three signal conditioner circuit cards (SCCC) and a BITE display card (BDC).

Fuel quantity shows on the common display system (CDS).

Signal Conditioner Circuit Cards

There is one SCCC for each fuel tank. Each SCCC has these functions:
Sends a low Z signal to the tank units and compensator
Reads the high Z return signal from the tank units and compensators
Calculates total fuel weight for its tank
Changes analog signals to ARINC 429 signals
Sends tank fuel weight data to the DEUs Sends real time fault data to the BDC.
The tank 2 SCCC also sends total fuel tank weight to the FMC. BITE Display Card

The BDC has these functions:
Keep fault data in non-volatile memory
Send a signal to each SCCC to start a real time test for fault isolation
Send and receives fault data with the flight management computer for fault isolation."

28-41-00-007
"FUEL INDICATING - FUEL QUANTITY INDICATIONS
General Individual fuel tank quantity shows on the CDS and the refuel panel.
Fuel quantity shows in pounds.

Fuel Configuration Messages

Fuel configuration messages show on the CDS. These messages show a problem with the configuration of the fuel system.

The LOW message shows that either main tank 1 or main tank 2 has less than 2,000 lb (907 kg) of fuel. This message goes away when there is more than 2,500 lb (1133 kg) of fuel in that tank. The low fuel condition must exist for 30 seconds before the LOW message shows.

The IMBAL message shows when there is a difference of 1,000 lb (453 kg) between main tank 1 and main tank 2. The message goes away when the difference between tanks is 200 lb (90 kg) or less. The IMBAL message only shows when the airplane is in the air. The IMBAL message does not show when the LOW message shows. The imbalance condition must exist for 60 seconds before the IMBAL message shows.

The CONFIG message shows when one of these conditions exist:
1,600 lb (725 kg) or more of fuel in the center tank
Both center tank boost pumps off
Either engine is in operation.
After the CONFIG message shows, it stays on until all of these conditions are true:
800 lb (360 kg) or less of fuel in the center tank
A minimum of one center tank pump is producing high pressure The two engines are not in operation."
:8 :rolleyes:

[avioniker]

I'm not "suggesting" anything. I simply cut and pasted from the available online manuals and used personal experience on the aircraft for the rest.

The gauges show all the measurable fuel in the tanks. There is no regard as to usability or unusability.

As to what customer code the information I cut and pasted comes from it's from generic SDS. The tanks and probes aren't altered for customer customization. The only changes are to software for indication in pounds or kilograms.

When the center tank indicates 0 the tank is virtually dry. I have pumped the fuel out of the tank to the 0 mark and gotten less than 5 gallons out of the sumps at the end.

With the plane flying there is a nose up attitude which keeps the pickups in the fuel. It's almost all usable (barring turbulence).

The tank units are simple capacitors which measure the reactance of their environment. They aren't calibrated to measure anything exotic and the processor doesn't have any setting for a usable or unusable quantity of fuel.

Each Signal Conditioner Circuit Card does, in fact, sum the total of the probe values to measure the total of each tank. Those quantities are summed to give a total fuel aboard value which is sent to the FMCS (if installed) for CG and economy management.

We're disagreeing for a very basic reason. You're looking at the operation manual and I'm dealing with technical specifics. The mechanical indicators of the past didn't have the capability of indicating quantity within a meaningful tolerance to the aircrew. With the advent of digital measurement the accuracy capability is better than 1% so the indications show it all. The crew needs to be aware of what portion of that fuel isn't usable.
The system measures all measurable fuel and that's just about all but the bottom 2" in the lowest portion of the lowest fuel cell compartment.

If the fuel indicator says 100lbs. That's what's in the tank.

[fruitloop]

Quote "When the center tank indicates 0 the tank is virtually dry. I have pumped the fuel out of the tank to the 0 mark and gotten less than 5 gallons out of the sumps at the end.

With the plane flying there is a nose up attitude which keeps the pickups in the fuel. It's almost all usable (barring turbulence).

Without meaning to burst ya bubble if you only got about 5 gallons out of the drain (who said it was at the lowest point of the tank on the ground i.e nose down attitude)put a nose support jack in position and level the A/C and see how much you get out.You may be very surprised how much is still there if you have to get into the centre tank..

[Sperm Bank]

Avionika thanks for that. Ozzies tend to get a little carried away at times quoting figures etc. From my experience in the a/c and by LISTENING to engineers who know alot more than me, I have come to the conclusion that the tanks are empty when the guage say empty.

If this was not the case, we would have to make the appropriate allowance when doing load sheet calcuations. If the fuel guages say we have seven tons, we have seven tons. Thats what goes on the load sheet and I suspect there would be hell to pay if this was not the case, in the unlikely event of an investigation into a incident/accident.

When the guage says zero, it means zero!!! Empty, nada, nil!!!!!!!

[Checkboard]

As a regulatory thing, US FAR 25.1553 (http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_14/14cfr25_00.html) states:

If the unusable fuel supply for any tank exceeds one gallon, or five percent of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight

Given that the 737 fuel guages don't have a red arc, they either don't show unuseable fuel, or that fuel is less than a gallon...

[avioniker]

Fruitloop,
Since we just completed the switch mods on our fleet I've had the opportunity to be in the center tank many, many times over the last year.
Like I said I base my statement on tech data and experience, not interpretation opinion.

[fruitloop]

avioniker It sounds like you also have had that pleasure (misfortune) of the centre tank "mods" .... (I'm getting to old for twisting) BTW ever had a code "5" and how did you clear it ??
cheers