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.