To think that I've worked with aircraft having a VARIABLE MZFW (B744, a fuel density thing), or NO MZFW AT ALL (DHC6 Twin Otter, a struts thing).
MZFW of course represents a manufacturer-defined limitation designed to prevent wing-fuselage joint over-stressing while operating the aircraft at maximum allowable loadfactor and at MTOW (structural), and is expressed in a (fuselage) weight limit.
The derived/designed MZFW represents a theoretical condition, and has no doubt resulted from a necessity to prevent mishandling.
Unless specifically stated otherwise in the Flight Manual, aircraft equipped with a CWT have defined/designed MZFW's that take a full CWT on departure into consideration. Contrary to ballast fuel, useable CWT fuel is therefor not part of any ZFW considerations.
Scavenge pumps INOP will cause a portion of CWT fuel to be un-useable, and therefor become ballast, however, no ZFW impact.
Non-standard fuselage AUX tanks (as in A319CJ), although representing useable fuel, should be considered to impact a ZFW.
The published MZFW should be lowered with the fuel mass loaded into these tanks when the MZFW of the original aircraft design/certification does not take fuselage AUX tank fuel into consideration, resulting in a lower max payload (pardon the pun).
... 4 pages of blissfull ignorance, blatant misconceptions, goodhearted explanations, and a lot of air- which includes this post, of course...
Cheers, Max.
