If the wing tanks are not full, the unused fuel in the center tank should be considered as “payload” and would count towards the Maximum Zero Fuel Weight limitation. The maximum zero fuel weight is a structural limitation of the wing. It’s basically given by how much weight you can hang from the wings inside the fuselage.
The AFM states that if there’s fuel in the center tanks and the wing tanks are not full then the sum of the zero fuel weight plus the weight of the unused fuel in the center tank could exceed the MZFW. If this happens, the AFM allows an exceedance of the MZFW of 2300Kg during take off and cruise and 1400Kg for landing.
The MZFW of the 737-700 is around 54657Kg. So the sum of the zero fuel weight plus the fuel in the center tanks whenever the wings aren’t full must be below 56975Kg for take off and cruise and below 56057Kg for landing.
This is not an issue if you follow the fuel usage guide which is simply:
-If there’s fuel in the center tank → 6 pumps on
-If there’s no fuel in the center tank → 4 pumps on
Then there’s the abnormal scenario of having fuel in the center tank and a malfunction of both center tank pumps. For that there’s a QHR Checklist called FUEL PUMP LOW PRESSURE that says among other things “Center tank fuel is unusable. Main tank fuel may not be sufficient for the planned flight”. So you’d need to run numbers to make a decision.
Sadly I cannot share with you the AFM but the QRH and FCOM that I linked are a must have for any PMDG user since Boeing revoked the permission to include these manuals with the addon purchase.
