Why do a loadsheet. We need to know how heavy the aircraft is on take off (TOW), so that we can use an appropriate amount of thrust for take off. We also have to check that the aircraft will not be above its maximum landing weight (MLW) when we reach the destination. We have to know that the payload is not above a maximum (MZFW) otherwise we will be above structural limits. Knowing how many people are on board is also required in the case of any incident. Knowing where the passengers are sitting and where the cargo is loaded is important for the trim (balance) of the aircraft.
The aircraft dispatcher (also called a red-cap) will prepare the loadsheet as he is reposible for ensuring that: the aircraft is fuelled, catered (bear in mind on a 737 the catering adds up to around 1000kg), loaded with the correct number of people, baggage, and freight.
Good luck with the studies!
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