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B747 MEL Main Tank Boost Pumps

Mon Feb 10, 2020 10:33 pm


I have question about those two MELs. I mean two because there is one for Main Tank 1 and 4 pumps and another for Main Tank 2 and 3 pumps.
I think I understand what to do with that but I quite do not understand why.

For engines 1 and 4. Why they have to be manifolded from CWT during take-off? Is it to ensure they fuel supply will be continuous during changes of attitude? I assume one pump inop so it could suck air from Main Tank? Is this the reason why there is minimum fuel in CWT required?

For engines 2 and 3. "Associated tank override/jettison pumps are selected on for takeoff". Does that mean that I feed engine 2 or 3 via crossfeed system? So the fuel is for engine 2 is from tank 2 but it goes through crossfeed? Is that even possible? I mean according to scheme it looks so.

Maybe I'm totally wrong. But I what I do not understand is why I have to include CWT fuel to ZFW. Well, I understand that in case of engines 1 and 4 I use non-standard distribution and I could have "empty" wings, fuel in CWT and if aircraft was loaded to MZFW the stress on wings would be too much. Sure, it makes sense. I think I could use restriction on ZFW too, it would do the same job.
But how does this work with engines 2 and 3? I can not find any connection in that.
I see that I have to keep fuel crossfeed valve open and this itself requires CWT empty or included in ZFW but still I do not know why. I would say that I can have fuel in CWT. I can use it and when it is empty I can switch to fuel from Main Tank but there is obviously something what I do not understand.
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Re: B747 MEL Main Tank Boost Pumps

Sat Feb 15, 2020 4:24 am

From distant memory...
For 1 and 4, there are 2 options in the MEL. Both cases are meant to protect the engine in case of failure of the second pump, basically to re-establish the redundancy that you lost with the failed pump.

Option 1 is to load the outer tank with the inop pump with enough fuel (11T) to ensure the engine can be gravity fed at high power and acceleration for takeoff. It also specifies a minimum tank level for landing (2T) for the same purpose, but lower as engine loads, accelerations and pitch angles are much lower on approach.

Option 2 allows the operator to use the CWT to ensure redundancy by loading it with enough fuel (7.7T) to allow for the center override pumps to feed 1 and 4 (as per normal ops when more than 7.7T are loaded in the center) during takeoff. The same 2T restriction applies for landing. As to why the center fuel should be considered part of the ZFW, I can't quite remember (since you can still use it normally), but it might have to do with catering for the lack of wing bending relief if you don't have full outer tanks (since this option allows you to avoid having to load 11T in the affected outer) or to cater for a quick return to land if you lose the other pump. Not sure, but likely bending relief as you mentioned.

For 2 and 3, the maintenance procedure is to deactivate the associated inner tank crossfeed valve to open. This allows the override pumps in the inner tanks to ensure proper feed (yes, through the crossfeed gallery). My guess is that the deactivated crossfeed would mess with the system logic and prevent it from using the center fuel as it normally would, i.e. on take off, these crossfeed valves would automatically close to allow the center tank to feed 1 and 4.
Note that the MEL contains more procedures associated with main pumps 2 and 3 inop, depending on whether the fwd or ft pump is inop. In these cases, the CWT limitation can be removed and replaced by a minimum fuel quantity in the tank for take off and landing.
Basically, the no-center-tank procedure would be used for low fuel loads as it doesn't have a minimum main tank qty for takeoff, and the other, more specific procedures can be used for high fuel loads as they allow usage of the CWT but require minimum qty in the main tanks for takeoff (35.5T). So there's a bit of flexibility for the operator.

There, clear as mud...
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