B747 Main tank boost pump MEL

[Rarife]

Hello,
I have question about MEL for B744. When Main tank boost pump is inop there is strange limitation for MZFW.

...
a) Prior to engine start, a minimum of
17,000 lb (7,711 kg) fuel is loaded in the
center wing tank and all center wing tank
fuel is included as part of Zero Fuel
Weight,
...

I do not get it. Well I do but I do not understand why. If there is 15 tons of fuel in center tank why should I lower the MZFW by this. I can use that fuel. It is not "dead" weight.
What was even more confusing was that our engineering department said there is no limitation and it is just info for captain.
I just do not know what is this point trying to say.

[bigduke6]

Originally Posted by Rarife

Hello,
I have question about MEL for B744. When Main tank boost pump is inop there is strange limitation for MZFW.


I do not get it. Well I do but I do not understand why. If there is 15 tons of fuel in center tank why should I lower the MZFW by this. I can use that fuel. It is not "dead" weight.
What was even more confusing was that our engineering department said there is no limitation and it is just info for captain.
I just do not know what is this point trying to say.

You can do the pump deferral 2 ways: have more fuel in the tank with the inop pump that will be considered unusable, or instead, as you are doing, have fuel in the CWT to feed the engines for take-off and then consider 15 tons of that as unusable, but then have less unusable fuel in the wing tank.

I believe it really comes down to what I think Boeing refers to as alternate certification. You think if you defer one main tank boost pump, no big deal, the other pump will work and you can get all of the fuel out of that under positive pressure, or worse case, you could suction feed it. And in-flight that should actually be the case if you departed with all the pumps working and 1 of then became inop during the flight. But since you are dispatching the plane with 1 pump inop, Boeing has to consider the other remaining pump failing, and then some of the fuel in that tank being unusable. That fuel would still suction feed, but they did not certify flying around on suction feed as a normal operation. And for normal operation, they were considering keeping the fuel balanced from side to side, etc.

So you cannot plan on using the fuel per the certification rules. But in an "emergency" the fuel is perfectly usable with no ill effect.

[Dave Therhino]

I suspect the 17,000 lbs minimum comes from a series of ADs issued to address various fuel pump failure modes and the potential for the pump to create an ignition source under dry running conditions. Those ADs required the pilots to monitor the fuel quantity and shut the center wing tank pumps off at a specified CWT quantity (7,000 lbs in climb or 3,000 lbs in cruise) before their inlets had the potential to be uncovered. The ADs required that, if you had any mission fuel loaded in the CWT, there must be a minimum of 17,000 lbs in that tank. That 17,000 lbs minimum was based on the normal fuel burn sequence using CWT fuel first, and was intended to ensure that the tank would not hit the 7,000 lb climb shutoff level during the high workload initial climb portion of the flight when the crew was unlikely to be able to effectively monitor the CWT level and shut the pumps off on time.

Here's an educated guess about the other aspects of the MMEL item: The MMEL entry appears to be trying to account for at least two scenarios with that limitation. The concern is that the single remaining main pump in the tank with the inoperative pump may also fail. 1) If that occurs early in the flight, it may necessitate a turnback or diversion due to trapped fuel in the affected tank (and in the opposite tank due to the imbalance limit), and therefore insufficient mission fuel. You may therefore be landing with nearly all of that center tank fuel, so they require you to treat it as part of the MZFW so you stay within landing weight limits if that scenario occurs. 2) If the second boost pump in the affected tank fails late in the mission when you are on tank to engine feed with equal amounts in the four mains, causing you to lose access to the fuel in the affected tank, they must have calculated you needed some contingency fuel in the center tank for that scenario, and it must have been less than 17,000 lbs. I highly suspect the 17,000 lb minimum would have been driven by the fuel pump ignition source issue I described above, because it would be an unusual coincidence for the contingency fuel to be the same 17,000 lbs amount.

[Long Haul]

The 744 is designed to only carry Center tank fuel when the main tanks are full. In this configuration the fuel in the wing tanks counter balances the wings' lifting force, enabling the center tank to carry a load which is burned off first before stressing the fuselage. If one of the main tank one or four pumps is u.s., you run the danger of uncovering the other pump during takeoff, so you need to maintain a minimum of 11,000 kg for takeoff per tank, which is obviously wasteful if you have a short leg but need to take 44,000 kg fuel due to this restriction. A way around this is to use the operative center pumps to feed engines one and four for takeoff by putting at least 7700 kg in the center tank, ensuring that the center tank feeds the outboard engines. This results, however, in a non-standard fuel distribution; i.e. center tank fuel loaded without the main tanks being full. So, at maximum zero fuel weight, you would have the wings producing a lot of lift, center tank weight exacerbating that effect, and less fuel than usual in the main tanks to counterbalance that force. Reducing the maximum zero fuel weight by the weight of the center tank fuel ensures that the structure will not become overstressed because the wings will be producing less lift, which then can be balanced by the lower than usual weight in the main tanks.

[back to Boeing]

Out of random curiosity. Does this restriction have anything to do with the TWA 800 accident. I accept that that may be lighting a candle to touch paper. But is it boeing covering its arse?

[Long Haul]

Originally Posted by back to Boeing

Out of random curiosity. Does this restriction have anything to do with the TWA 800 accident. I accept that that may be lighting a candle to touch paper. But is it boeing covering its arse?

No, it has nothing to do with that. The amount (7700 kg) ensures that the outside engines continue to get fed by the center tank throughout the takeoff and climb. The fact that the maximum zero fuel weight needs to be reduced is true whenever there is center fuel tank present but the main tanks aren't full.

[Rarife]

Great, thank you. I was quite busy so I'm late.

But still. I got what you say and it makes sense to me but as I said. There was no restriction on that flight. Definitely not that much. I remember that because we would have to leave all cargo on ground and it did not happen. They had about 110 tons in wings and about 25 tons in CWT. And engineering confirmed that there is no restriction and it is just "info".

And as I wrote it I got it now...maybe.

Short flight but I need fuel in center tank to feed engines. So I must reduce MZFW to counterbalance.
Long flight. My case. No problem because wings are full and other fuel in center tank so the aircraft is loaded and stressed normaly.