Flyman35,
We have 2 variants of weight for the airline I fly for, 73500kg and 77000kgs as the maximum structural take off weight.
As has been mentioned you cannot exceed the maximum structural take off weight, however there are instances where you may be limited to a weight that is below your maximum take off weight, this is referred to as a regulated take off weight (RTOW). For example you may be at an airfield in the summer, that is 2000ft above sea level with a temperature of 30 degrees giving you a high density altitude. In this situtation you may be limited to a weight in the region of say, 69000kgs. If you have a high zero fuel weight, and need to carry alot of fuel to get to your destination you may find that loading the required block fuel would exceed this weight. In this situation you have various options.
Lets say your planned take off weight is 69834kgs and your RTOW is 69000kgs. There are a number of things you can do.
1. Simply reduce the weight, either through off loading PAX, freight, or baggage, not the desired option!
2. Increasing the RTOW by doing QNH corrections. With very high pressure you can sometimes increase the weight by 600kg.
3. Increasing the RTOW by doing a packs off take off, or leave the APU running with the APU bleed on, which will supply the packs.
4. Take credit for a headwind.
5. Reduce the fuel load. If you can meet certain criteria you can reduce the contingency fuel to 3%. Also, if the weather meets what is required in the company operations manual, you can not take alternate fuel, however you must carry addtional fuel, which sometimes saves you a few hundred kilograms.
Hope this helps.