The 747-400 will take weight penalties when SYD
has bad weather and the airplane has to carry enough fuel for MEL
as an alternate. This can happen a little more often than you'd think since SYD
gets limited to CAT I operations which means it does not take that many or the possibility of morning fog to require the extra fuel. The higher MTOW 747-400ER with the extra 20K lbs of payload available is just enough to make MEL
with no weight restrictions, so if it is used on a flight to SYD
, there will not be weight restrictions at all even if there is bad weather.
If the weather is good, planes usually do not take weight restrictions. With a full load of pax, and cargo the airplanes will be going out at MTOW so it can get tight as there is not a lot of extra margin. However with an 80% load factor, there are no problems.
If you have never designed an airplane part before, let the real designers do the work!